This explainer walks through the assembly-level definition of a component and the proposed embedding of components into native JavaScript runtimes. For a more user-focused explanation, take a look at the Component Model Documentation.
- Gated features
- Grammar
- Component invariants
- JavaScript embedding
- Examples
- TODO
By default, the features described in this explainer (as well as the supporting Binary.md, WIT.md and CanonicalABI.md) have been implemented and are included in the WASI Preview 2 stability milestone. Features that are not part of Preview 2 are demarcated by one of the emoji symbols listed below; these emojis will be removed once they are implemented, considered stable and included in a future milestone:
- 🪙: value imports/exports and component-level start function
- 🪺: nested namespaces and packages in import/export names
- 🧵: threading built-ins
(Based on the previous scoping and layering proposal to the WebAssembly CG, this repo merges and supersedes the module-linking and interface-types proposals, pushing some of their original features into the post-MVP future feature backlog.)
This section defines components using an EBNF grammar that parses something in between a pure Abstract Syntax Tree (like the Core WebAssembly spec's Structure Section) and a complete text format (like the Core WebAssembly spec's Text Format Section). The goal is to balance completeness with succinctness, with just enough detail to write examples and define a binary format in the style of the Binary Format Section, deferring full precision to the formal specification.
The main way the grammar hand-waves is regarding definition uses, where indices
referring to X definitions (written <Xidx>) should, in the real text
format, explicitly allow identifiers (<id>), checking at parse time that the
identifier resolves to an X definition and then embedding the resolved index
into the AST.
Additionally, standard abbreviations defined by the Core WebAssembly text format (e.g., inline export definitions) are assumed but not explicitly defined below.
At the top-level, a component is a sequence of definitions of various kinds:
component ::= (component <id>? <definition>*)
definition ::= core-prefix(<core:module>)
| core-prefix(<core:instance>)
| core-prefix(<core:type>)
| <component>
| <instance>
| <alias>
| <type>
| <canon>
| <start> 🪺
| <import>
| <export>
| <value> 🪙
where core-prefix(X) parses '(' 'core' Y ')' when X parses '(' Y ')'Components are like Core WebAssembly modules in that their contained definitions are acyclic: definitions can only refer to preceding definitions (in the AST, text format and binary format). However, unlike modules, components can arbitrarily interleave different kinds of definitions.
The core-prefix meta-function transforms a grammatical rule for parsing a
Core WebAssembly definition into a grammatical rule for parsing the same
definition, but with a core token added right after the leftmost paren.
For example, core:module accepts (module (func)) so
core-prefix(<core:module>) accepts (core module (func)). Note that the
inner func doesn't need a core prefix; the core token is used to mark the
transition from parsing component definitions into core definitions.
The core:module production is unmodified by the Component Model and thus
components embed Core WebAssembly (text and binary format) modules as currently
standardized, allowing reuse of an unmodified Core WebAssembly implementation.
The next production, core:instance, is not currently included in Core
WebAssembly, but would be if Core WebAssembly adopted the module-linking
proposal. This new core definition is introduced below, alongside its
component-level counterpart. Finally, the existing core:type production is
extended below to add core module types as proposed for module-linking. Thus,
the overall idea is to represent core definitions (in the AST, binary and text
format) as-if they had already been added to Core WebAssembly so that, if they
eventually are, the implementation of decoding and validation can be shared in
a layered fashion.
The next kind of definition is, recursively, a component itself. Thus, components form trees with all other kinds of definitions only appearing at the leaves. For example, with what's defined so far, we can write the following component:
(component
(component
(core module (func (export "one") (result i32) (i32.const 1)))
(core module (func (export "two") (result f32) (f32.const 2)))
)
(core module (func (export "three") (result i64) (i64.const 3)))
(component
(component
(core module (func (export "four") (result f64) (f64.const 4)))
)
)
(component)
)This top-level component roots a tree with 4 modules and 1 component as
leaves. However, in the absence of any instance definitions (introduced
next), nothing will be instantiated or executed at runtime; everything here is
dead code.
Like Core WebAssembly, the Component Model places each
definition into one of a fixed set of index spaces, allowing the
definition to be referred to by subsequent definitions (in the text and binary
format) via a nonnegative integral index. When defining, validating and
executing a component, there are 5 component-level index spaces:
- (component) functions
- (component) values
- (component) types
- component instances
- components
5 core index spaces that also exist in WebAssembly 1.0:
- (core) functions
- (core) tables
- (core) memories
- (core) globals
- (core) types
and 2 additional core index spaces that contain core definition introduced by the Component Model that are not in WebAssembly 1.0 (yet: the module-linking proposal would add them):
- module instances
- modules
for a total of 12 index spaces that need to be maintained by an implementation
when, e.g., validating a component. These 12 index spaces correspond 1:1 with
the terminals of the sort production defined below and thus "sort" and
"index space" can be used interchangeably.
Also like Core WebAssembly, the Component Model text format allows identifiers to be used in place of these indices, which are resolved when parsing into indices in the AST (upon which validation and execution is defined). Thus, the following two components are equivalent:
(component
(core module (; empty ;))
(component (; empty ;))
(core module (; empty ;))
(export "C" (component 0))
(export "M1" (core module 0))
(export "M2" (core module 1))
)(component
(core module $M1 (; empty ;))
(component $C (; empty ;))
(core module $M2 (; empty ;))
(export "C" (component $C))
(export "M1" (core module $M1))
(export "M2" (core module $M2))
)Whereas modules and components represent immutable code, instances associate code with potentially-mutable state (e.g., linear memory) and thus are necessary to create before being able to run the code. Instance definitions create module or component instances by selecting a module or component and then supplying a set of named arguments which satisfy all the named imports of the selected module or component.
The syntax for defining a core module instance is:
core:instance ::= (instance <id>? <core:instancexpr>)
core:instanceexpr ::= (instantiate <core:moduleidx> <core:instantiatearg>*)
| <core:inlineexport>*
core:instantiatearg ::= (with <core:name> (instance <core:instanceidx>))
| (with <core:name> (instance <core:inlineexport>*))
core:sortidx ::= (<core:sort> <u32>)
core:sort ::= func
| table
| memory
| global
| type
| module
| instance
core:inlineexport ::= (export <core:name> <core:sortidx>)When instantiating a module via instantiate, the two-level imports of the
core modules are resolved as follows:
- The first
core:nameof the import is looked up in the named list ofcore:instantiateargto select a core module instance. (In the future, othercore:sorts could be allowed if core wasm adds single-level imports.) - The second
core:nameof the import is looked up in the named list of exports of the core module instance found by the first step to select the imported core definition.
Each core:sort corresponds 1:1 with a distinct index space that contains
only core definitions of that sort. The u32 field of core:sortidx
indexes into the sort's associated index space to select a definition.
Based on this, we can link two core modules $A and $B together with the
following component:
(component
(core module $A
(func (export "one") (result i32) (i32.const 1))
)
(core module $B
(func (import "a" "one") (result i32))
)
(core instance $a (instantiate $A))
(core instance $b (instantiate $B (with "a" (instance $a))))
)To see examples of other sorts, we'll need alias definitions, which are
introduced in the next section.
The <core:inlineexport>* form of core:instanceexpr allows module instances
to be created by directly tupling together preceding definitions, without the
need to instantiate a helper module. The <core:inlineexport>* form of
core:instantiatearg is syntactic sugar that is expanded during text format
parsing into an out-of-line instance definition referenced by with. To show
an example of these, we'll also need the alias definitions introduced in the
next section.
The syntax for defining component instances is symmetric to core module
instances, but with an expanded component-level definition of sort:
instance ::= (instance <id>? <instanceexpr>)
instanceexpr ::= (instantiate <componentidx> <instantiatearg>*)
| <inlineexport>*
instantiatearg ::= (with <name> <sortidx>)
| (with <name> (instance <inlineexport>*))
name ::= <core:name>
sortidx ::= (<sort> <u32>)
sort ::= core <core:sort>
| func
| value 🪙
| type
| component
| instance
inlineexport ::= (export <exportname> <sortidx>)Because component-level function, type and instance definitions are different
than core-level function, type and instance definitions, they are put into
disjoint index spaces which are indexed separately. Components may import
and export various core definitions (when they are compatible with the
shared-nothing model, which currently means only module, but may in the
future include data). Thus, component-level sort injects the full set
of core:sort, so that they may be referenced (leaving it up to validation
rules to throw out the core sorts that aren't allowed in various contexts).
The name production reuses the core:name quoted-string-literal syntax of
Core WebAssembly (which appears in core module imports and exports and can
contain any valid UTF-8 string).
🪙 The value sort refers to a value that is provided and consumed during
instantiation. How this works is described in the
value definitions section.
To see a non-trivial example of component instantiation, we'll first need to introduce a few other definitions below that allow components to import, define and export component functions.
Alias definitions project definitions out of other components' index spaces and
into the current component's index spaces. As represented in the AST below,
there are three kinds of "targets" for an alias: the export of a component
instance, the core export of a core module instance and a definition of an
outer component (containing the current component):
alias ::= (alias <aliastarget> (<sort> <id>?))
aliastarget ::= export <instanceidx> <name>
| core export <core:instanceidx> <core:name>
| outer <u32> <u32>If present, the id of the alias is bound to the new index added by the alias
and can be used anywhere a normal id can be used.
In the case of export aliases, validation ensures name is an export in the
target instance and has a matching sort.
In the case of outer aliases, the u32 pair serves as a de Bruijn
index, with first u32 being the number of enclosing components/modules to
skip and the second u32 being an index into the target's sort's index space.
In particular, the first u32 can be 0, in which case the outer alias refers
to the current component. To maintain the acyclicity of module instantiation,
outer aliases are only allowed to refer to preceding outer definitions.
Components containing outer aliases effectively produce a closure at
instantiation time, including a copy of the outer-aliased definitions. Because
of the prevalent assumption that components are immutable values, outer aliases
are restricted to only refer to immutable definitions: non-resource types,
modules and components. (In the future, outer aliases to all sorts of
definitions could be allowed by recording the statefulness of the resulting
component in its type via some kind of "stateful" type attribute.)
Both kinds of aliases come with syntactic sugar for implicitly declaring them inline:
For export aliases, the inline sugar extends the definition of sortidx
and the various sort-specific indices:
sortidx ::= (<sort> <u32>) ;; as above
| <inlinealias>
Xidx ::= <u32> ;; as above
| <inlinealias>
inlinealias ::= (<sort> <u32> <name>+)If <sort> refers to a <core:sort>, then the <u32> of inlinealias is a
<core:instanceidx>; otherwise it's an <instanceidx>. For example, the
following snippet uses two inline function aliases:
(instance $j (instantiate $J (with "f" (func $i "f"))))
(export "x" (func $j "g" "h"))which are desugared into:
(alias export $i "f" (func $f_alias))
(instance $j (instantiate $J (with "f" (func $f_alias))))
(alias export $j "g" (instance $g_alias))
(alias export $g_alias "h" (func $h_alias))
(export "x" (func $h_alias))For outer aliases, the inline sugar is simply the identifier of the outer
definition, resolved using normal lexical scoping rules. For example, the
following component:
(component
(component $C ...)
(component
(instance (instantiate $C))
)
)is desugared into:
(component $Parent
(component $C ...)
(component
(alias outer $Parent $C (component $Parent_C))
(instance (instantiate $Parent_C))
)
)Lastly, for symmetry with imports, aliases can be written in an inverted form that puts the sort first:
(func $f (import "i" "f") ...type...) ≡ (import "i" "f" (func $f ...type...)) (WebAssembly 1.0)
(func $f (alias export $i "f")) ≡ (alias export $i "f" (func $f))
(core module $m (alias export $i "m")) ≡ (alias export $i "m" (core module $m))
(core func $f (alias core export $i "f")) ≡ (alias core export $i "f" (core func $f))With what's defined so far, we're able to link modules with arbitrary renamings:
(component
(core module $A
(func (export "one") (result i32) (i32.const 1))
(func (export "two") (result i32) (i32.const 2))
(func (export "three") (result i32) (i32.const 3))
)
(core module $B
(func (import "a" "one") (result i32))
)
(core instance $a (instantiate $A))
(core instance $b1 (instantiate $B
(with "a" (instance $a)) ;; no renaming
))
(core func $a_two (alias core export $a "two")) ;; ≡ (alias core export $a "two" (core func $a_two))
(core instance $b2 (instantiate $B
(with "a" (instance
(export "one" (func $a_two)) ;; renaming, using out-of-line alias
))
))
(core instance $b3 (instantiate $B
(with "a" (instance
(export "one" (func $a "three")) ;; renaming, using <inlinealias>
))
))
)To show analogous examples of linking components, we'll need component-level type and function definitions which are introduced in the next two sections.
The syntax for defining core types extends the existing core type definition
syntax, adding a module type constructor:
core:rectype ::= ... from the Core WebAssembly spec
core:typedef ::= ... from the Core WebAssembly spec
core:subtype ::= ... from the Core WebAssembly spec
core:comptype ::= ... from the Core WebAssembly spec
| <core:moduletype>
core:moduletype ::= (module <core:moduledecl>*)
core:moduledecl ::= <core:importdecl>
| <core:type>
| <core:alias>
| <core:exportdecl>
core:alias ::= (alias <core:aliastarget> (<core:sort> <id>?))
core:aliastarget ::= outer <u32> <u32>
core:importdecl ::= (import <core:name> <core:name> <core:importdesc>)
core:exportdecl ::= (export <core:name> <core:exportdesc>)
core:exportdesc ::= strip-id(<core:importdesc>)
where strip-id(X) parses '(' sort Y ')' when X parses '(' sort <id>? Y ')'Here, core:comptype (short for "composite type") as defined in the GC
proposal is extended with a module type constructor. The GC proposal also
adds recursion and explicit subtyping between core wasm types. Owing to
their different requirements and intended modes of usage, module types
support implicit subtyping and are not recursive. Thus, the existing core
validation rules would require the declared supertypes of module types to be
empty and disallow recursive use of module types.
In the MVP, validation will also reject core:moduletype defining or aliasing
other core:moduletypes, since, before module-linking, core modules cannot
themselves import or export other core modules.
The body of a module type contains an ordered list of "module declarators"
which describe, at a type level, the imports and exports of the module. In a
module-type context, import and export declarators can both reuse the existing
core:importdesc production defined in WebAssembly 1.0, with the only
difference being that, in the text format, core:importdesc can bind an
identifier for later reuse while core:exportdesc cannot.
With the Core WebAssembly type-imports, module types will need the ability to
define the types of exports based on the types of imports. In preparation for
this, module types start with an empty type index space that is populated by
type declarators, so that, in the future, these type declarators can refer to
type imports local to the module type itself. For example, in the future, the
following module type would be expressible:
(component $C
(core type $M (module
(import "" "T" (type $T))
(type $PairT (struct (field (ref $T)) (field (ref $T))))
(export "make_pair" (func (param (ref $T)) (result (ref $PairT))))
))
)In this example, $M has a distinct type index space from $C, where element
0 is the imported type, element 1 is the struct type, and element 2 is an
implicitly-created func type referring to both.
Lastly, the core:alias module declarator allows a module type definition to
reuse (rather than redefine) type definitions in the enclosing component's core
type index space via outer type alias. In the MVP, validation restricts
core:alias module declarators to only allow outer type aliases (into an
enclosing component's or component-type's core type index space). In the
future, more kinds of aliases would be meaningful and allowed.
As an example, the following component defines two semantically-equivalent
module types, where the former defines the function type via type declarator
and the latter refers via alias declarator.
(component $C
(core type $C1 (module
(type (func (param i32) (result i32)))
(import "a" "b" (func (type 0)))
(export "c" (func (type 0)))
))
(core type $F (func (param i32) (result i32)))
(core type $C2 (module
(alias outer $C $F (type))
(import "a" "b" (func (type 0)))
(export "c" (func (type 0)))
))
)Component-level type definitions are symmetric to core-level type definitions,
but use a completely different set of value types. Unlike core:valtype
which is low-level and assumes a shared linear memory for communicating
compound values, component-level value types assume no shared memory and must
therefore be high-level, describing entire compound values.
type ::= (type <id>? <deftype>)
deftype ::= <defvaltype>
| <resourcetype>
| <functype>
| <componenttype>
| <instancetype>
defvaltype ::= bool
| s8 | u8 | s16 | u16 | s32 | u32 | s64 | u64
| f32 | f64
| char | string
| (record (field "<label>" <valtype>)+)
| (variant (case "<label>" <valtype>?)+)
| (list <valtype>)
| (tuple <valtype>+)
| (flags "<label>"+)
| (enum "<label>"+)
| (option <valtype>)
| (result <valtype>? (error <valtype>)?)
| (own <typeidx>)
| (borrow <typeidx>)
valtype ::= <typeidx>
| <defvaltype>
resourcetype ::= (resource (rep i32) (dtor <funcidx>)?)
functype ::= (func <paramlist> <resultlist>)
paramlist ::= (param "<label>" <valtype>)*
resultlist ::= (result "<label>" <valtype>)*
| (result <valtype>)
componenttype ::= (component <componentdecl>*)
instancetype ::= (instance <instancedecl>*)
componentdecl ::= <importdecl>
| <instancedecl>
instancedecl ::= core-prefix(<core:type>)
| <type>
| <alias>
| <exportdecl>
| <value> 🪙
importdecl ::= (import <importname> bind-id(<externdesc>))
exportdecl ::= (export <exportname> bind-id(<externdesc>))
externdesc ::= (<sort> (type <u32>) )
| core-prefix(<core:moduletype>)
| <functype>
| <componenttype>
| <instancetype>
| (value <valuebound>) 🪙
| (type <typebound>)
typebound ::= (eq <typeidx>)
| (sub resource)
valuebound ::= (eq <valueidx>) 🪙
| <valtype> 🪙
where bind-id(X) parses '(' sort <id>? Y ')' when X parses '(' sort Y ')'Because there is nothing in this type grammar analogous to the gc proposal's
rectype, none of these types are recursive.
The value types in valtype can be broken into two categories: fundamental
value types and specialized value types, where the latter are defined by
expansion into the former. The fundamental value types have the following
sets of abstract values:
| Type | Values |
|---|---|
bool |
true and false |
s8, s16, s32, s64 |
integers in the range [-2N-1, 2N-1-1] |
u8, u16, u32, u64 |
integers in the range [0, 2N-1] |
f32, f64 |
IEEE754 floating-point numbers, with a single NaN value |
char |
Unicode Scalar Values |
record |
heterogeneous tuples of named values |
variant |
heterogeneous tagged unions of named values |
list |
homogeneous, variable-length sequences of values |
own |
a unique, opaque address of a resource that will be destroyed when this value is dropped |
borrow |
an opaque address of a resource that must be dropped before the current export call returns |
How these abstract values are produced and consumed from Core WebAssembly
values and linear memory is configured by the component via canonical lifting
and lowering definitions, which are introduced below.
For example, while abstract variants contain a list of cases labelled by
name, canonical lifting and lowering map each case to an i32 value starting
at 0.
While core numeric types are defined in terms of sets of bit-patterns and operations that interpret the bits in various ways, component-level numeric types are defined in terms of sets of values. This allows the values to be translated between source languages and protocols that use different value representations.
Core integer types are just bit-patterns that don't distinguish between signed
and unsigned, while component-level integer types are sets of integers that
either include negative values or don't. Core floating-point types have many
distinct NaN bit-patterns, while component-level floating-point types have only
a single NaN value. And boolean values in core wasm are usually represented as
i32s where operations interpret all-zeros as false, while at the
component-level there is a bool type with true and false values.
The record, variant, and list types allow for grouping, categorizing,
and sequencing contained values.
The own and borrow value types are both handle types. Handles logically
contain the opaque address of a resource and avoid copying the resource when
passed across component boundaries. By way of metaphor to operating systems,
handles are analogous to file descriptors, which are stored in a table and may
only be used indirectly by untrusted user-mode processes via their integer
index in the table.
In the Component Model, handles are lifted-from and lowered-into i32 values
that index an encapsulated per-component-instance handle table that is
maintained by the canonical function definitions described
below. In the future, handles could be
backwards-compatibly lifted and lowered from [reference types] (via the
addition of a new canonopt, as introduced below).
The uniqueness and dropping conditions mentioned above are enforced at runtime
by the Component Model through these canonical definitions. The typeidx
immediate of a handle type must refer to a resource type (described below)
that statically classifies the particular kinds of resources the handle can
point to.
The sets of values allowed for the remaining specialized value types are defined by the following mapping:
(tuple <valtype>*) ↦ (record (field "𝒊" <valtype>)*) for 𝒊=0,1,...
(flags "<label>"*) ↦ (record (field "<label>" bool)*)
(enum "<label>"+) ↦ (variant (case "<label>")+)
(option <valtype>) ↦ (variant (case "none") (case "some" <valtype>))
(result <valtype>? (error <valtype>)?) ↦ (variant (case "ok" <valtype>?) (case "error" <valtype>?))
string ↦ (list char)
Specialized value types have the same set of semantic values as their
corresponding despecialized types, but have distinct type constructors
(which are not type-equal to the unspecialized type constructors) and
thus have distinct binary encodings. This allows specialized value types to
convey a more specific intent. For example, result isn't just a variant,
it's a variant that means success or failure, so source-code bindings
can expose it via idiomatic source-language error reporting. Additionally,
this can sometimes allow values to be represented differently. For example,
string in the Canonical ABI uses various Unicode encodings while
list<char> uses a sequence of 4-byte char code points. Similarly,
flags in the Canonical ABI uses a bit-vector while an equivalent record
of boolean fields uses a sequence of boolean-valued bytes.
Note that, at least initially, variants are required to have a non-empty list of
cases. This could be relaxed in the future to allow an empty list of cases, with
the empty (variant) effectively serving as an empty type and indicating
unreachability.
The remaining 4 type constructors in deftype use valtype to describe
shared-nothing functions, resources, components, and component instances:
The func type constructor describes a component-level function definition
that takes and returns a list of valtype. In contrast to core:functype,
the parameters and results of functype can have associated names which
validation requires to be unique. To improve the ergonomics and performance of
the common case of single-value-returning functions, function types may
additionally have a single unnamed return type. For this special case, bindings
generators are naturally encouraged to return the single value directly without
wrapping it in any containing record/object/struct.
The resource type constructor creates a fresh type for each instance of the
containing component (with "freshness" and its interaction with general
type-checking described in more detail below). Resource types
can be referred to by handle types (such as own and borrow) as well as the
canonical built-ins described below. The rep
immediate of a resource type specifies its core representation type, which
is currently fixed to i32, but will be relaxed in the future (to at least
include i64, but also potentially other types). When the last handle to a
resource is dropped, the resource's destructor function specified by the dtor
immediate will be called (if present), allowing the implementing component to
perform clean-up like freeing linear memory allocations.
The instance type constructor describes a list of named, typed definitions
that can be imported or exported by a component. Informally, instance types
correspond to the usual concept of an "interface" and instance types thus serve
as static interface descriptions. In addition to the S-Expression text format
defined here, which is meant to go inside component definitions, interfaces can
also be defined as standalone, human-friendly text files in the wit
Interface Definition Language.
The component type constructor is symmetric to the core module type
constructor and contains two lists of named definitions for the imports
and exports of a component, respectively. As suggested above, instance types
can show up in both the import and export types of a component type.
Both instance and component type constructors are built from a sequence of
"declarators", of which there are four kinds—type, alias, import and
export—where only component type constructors can contain import
declarators. The meanings of these declarators is basically the same as the
core module declarators introduced above, but expanded to cover the additional
capabilities of the component model.
The importdecl and exportdecl declarators correspond to component import
and export definitions, respectively, allowing an identifier to be bound for
use by subsequent declarators. The definitions of label, importname and
exportname are given in the imports and exports
section below. Following the precedent of core:typeuse, the text format
allows both references to out-of-line type definitions (via (type <typeidx>))
and inline type expressions that the text format desugars into out-of-line type
definitions.
🪙 The value case of externdesc describes a runtime value that is imported or
exported at instantiation time as described in the
value definitions section below.
The type case of externdesc describes an imported or exported type along
with its "bound":
The sub bound declares that the imported/exported type is an abstract type
which is a subtype of some other type. Currently, the only supported bound is
resource which (following the naming conventions of the GC proposal) means
"any resource type". Thus, only resource types can be imported/exported
abstractly, not arbitrary value types. This allows type imports to always be
compiled independently of their arguments using a "universal representation" for
handle values (viz., i32, as defined by the Canonical ABI).
In the future, sub may be extended to allow referencing other resource types,
thereby allowing abstract resource subtyping.
The eq bound says that the imported/exported type must be structurally equal
to some preceding type definition. This allows:
- an imported abstract type to be re-exported;
- components to introduce another label for a preceding abstract type (which can be necessary when implementing multiple independent interfaces with the same resource); and
- components to attach transparent type aliases to structural types to be
reflected in source-level bindings (e.g.,
(export "bytes" (type (eq (list u64))))could generate in C++ atypedef std::vector<uint64_t> bytesor in JS an exported field namedbytesthat aliasesUint64Array.
Relaxing the restrictions of core:alias declarators mentioned above, alias
declarators allow both outer and export aliases of type and instance
sorts. This allows the type exports of instance-typed import and export
declarators to be used by subsequent declarators in the type:
(component
(import "fancy-fs" (instance $fancy-fs
(export $fs "fs" (instance
(export "file" (type (sub resource)))
;; ...
))
(alias export $fs "file" (type $file))
(export "fancy-op" (func (param "f" (borrow $file))))
))
)The type declarator is restricted by validation to disallow resource type
definitions, thereby preventing "private" resource type definitions from
appearing in component types and avoiding the avoidance problem. Thus, the
only resource types possible in an instancetype or componenttype are
introduced by importdecl or exportdecl.
With what's defined so far, we can define component types using a mix of type definitions:
(component $C
(type $T (list (tuple string bool)))
(type $U (option $T))
(type $G (func (param "x" (list $T)) (result $U)))
(type $D (component
(alias outer $C $T (type $C_T))
(type $L (list $C_T))
(import "f" (func (param "x" $L) (result (list u8))))
(import "g" (func (type $G)))
(export "g2" (func (type $G)))
(export "h" (func (result $U)))
(import "T" (type $T (sub resource)))
(import "i" (func (param "x" (list (own $T)))))
(export "T2" (type $T' (eq $T)))
(export "U" (type $U' (sub resource)))
(export "j" (func (param "x" (borrow $T')) (result (own $U'))))
))
)Note that the inline use of $G and $U are syntactic sugar for outer
aliases.
Like core modules, components have an up-front validation phase in which the
definitions of a component are checked for basic consistency. Type checking
is a central part of validation and, e.g., occurs when validating that the
with arguments of an instantiate expression are
type-compatible with the imports of the component being instantiated.
To incrementally describe how type-checking works, we'll start by asking how type equality works for non-resource, non-handle, local type definitions and build up from there.
Type equality for almost all types (except as described below) is purely
structural. In a structural setting, types are considered to be Abstract
Syntax Trees whose nodes are type constructors with types like u8 and
string considered to be "nullary" type constructors that appear at leaves and
non-nullary type constructors like list and record appearing at parent
nodes. Then, type equality is defined to be AST equality. Importantly, these
type ASTs do not contain any type indices or depend on index space layout;
these binary format details are consumed by decoding to produce the AST. For
example, in the following compound component:
(component $A
(type $ListString1 (list string))
(type $ListListString1 (list $ListString1))
(type $ListListString2 (list $ListString1))
(component $B
(type $ListString2 (list string))
(type $ListListString3 (list $ListString2))
(type $ListString3 (alias outer $A $ListString1))
(type $ListListString4 (list $ListString3))
(type $ListListString5 (alias outer $A $ListListString1))
)
)all 5 variations of $ListListStringX are considered equal since, after
decoding, they all have the same AST.
Next, the type equality relation on ASTs is relaxed to a more flexible
subtyping relation. Currently, subtyping is only relaxed for instance and
component types, but may be relaxed for more type constructors in the future
to better support API Evolution (being careful to understand how subtyping
manifests itself in the wide variety of source languages so that
subtype-compatible updates don't inadvertantly break source-level clients).
Component and instance subtyping allows a subtype to export more and import
less than is declared by the supertype, ignoring the exact order of imports and
exports and considering only names. For example, here, $I1 is a subtype of
$I2:
(component
(type $I1 (instance
(export "foo" (func))
(export "bar" (func))
(export "baz" (func))
))
(type $I2 (instance
(export "bar" (func))
(export "foo" (func))
))
)and $C1 is a subtype of $C2:
(component
(type $C1 (component
(import "a" (func))
(export "x" (func))
(export "y" (func))
))
(type $C2 (component
(import "a" (func))
(import "b" (func))
(export "x" (func))
))
)When we next consider type imports and exports, there are two distinct
subcases of typebound to consider: eq and sub.
The eq bound adds a type equality rule (extending the built-in set of
subtyping rules mentioned above) saying that the imported type is structurally
equivalent to the type referenced in the bound. For example, in the component:
(component
(type $L1 (list u8))
(import "L2" (type $L2 (eq $L1)))
(import "L3" (type $L2 (eq $L1)))
(import "L4" (type $L2 (eq $L3)))
)all four $L* types are equal (in subtyping terms, they are all subtypes of
each other).
In contrast, the sub bound introduces a new abstract type which the rest of
the component must conservatively assume can be any type that is a subtype of
the bound. What this means for type-checking is that each subtype-bound type
import/export introduces a fresh abstract type that is unequal to every
preceding type definition. Currently (and likely in the MVP), the only
supported type bound is resource (which means "any resource type") and thus
the only abstract types are abstract resource types. As an example, in the
following component:
(component
(import "T1" (type $T1 (sub resource)))
(import "T2" (type $T2 (sub resource)))
)the types $T1 and $T2 are not equal.
Once a type is imported, it can be referred to by subsequent equality-bound type imports, thereby adding more types that it is equal to. For example, in the following component:
(component $C
(import "T1" (type $T1 (sub resource)))
(import "T2" (type $T2 (sub resource)))
(import "T3" (type $T3 (eq $T2)))
(type $ListT1 (list (own $T1)))
(type $ListT2 (list (own $T2)))
(type $ListT3 (list (own $T3)))
)the types $T2 and $T3 are equal to each other but not to $T1. By the
above transitive structural equality rules, the types $List2 and $List3 are
equal to each other but not to $List1.
Handle types (own and borrow) are structural types (like list) but, since
they refer to resource types, transitively "inherit" the freshness of abstract
resource types. For example, in the following component:
(component
(import "T" (type $T (sub resource)))
(import "U" (type $U (sub resource)))
(type $Own1 (own $T))
(type $Own2 (own $T))
(type $Own3 (own $U))
(type $ListOwn1 (list $Own1))
(type $ListOwn2 (list $Own2))
(type $ListOwn3 (list $Own3))
(type $Borrow1 (borrow $T))
(type $Borrow2 (borrow $T))
(type $Borrow3 (borrow $U))
(type $ListBorrow1 (list $Borrow1))
(type $ListBorrow2 (list $Borrow2))
(type $ListBorrow3 (list $Borrow3))
)the types $Own1 and $Own2 are equal to each other but not to $Own3 or
any of the $Borrow*. Similarly, $Borrow1 and $Borrow2 are equal to
each other but not $Borrow3. Transitively, the types $ListOwn1 and
$ListOwn2 are equal to each other but not $ListOwn3 or any of the
$ListBorrow*. These type-checking rules for type imports mirror the
introduction rule of universal types (∀T).
The above examples all show abstract types in terms of imports, but the same "freshness" condition applies when aliasing the exports of another component as well. For example, in this component:
(component
(import "C" (component $C
(export "T1" (type (sub resource)))
(export "T2" (type $T2 (sub resource)))
(export "T3" (type (eq $T2)))
))
(instance $c (instantiate $C))
(alias export $c "T1" (type $T1))
(alias export $c "T2" (type $T2))
(alias export $c "T3" (type $T3))
)the types $T2 and $T3 are equal to each other but not to $T1. These
type-checking rules for aliases of type exports mirror the elimination rule
of existential types (∃T).
Next, we consider resource type definitions which are a third source of abstract types. Unlike the abstract types introduced by type imports and exports, resource type definitions provide canonical built-ins for setting and getting a resource's private representation value (that are introduced below). These built-ins are necessarily scoped to the component instance that generated the resource type, thereby hiding access to a resource type's representation from the outside world. Because each component instantiation generates fresh resource types distinct from all preceding instances of the same component, resource types are ["generative"].
For example, in the following example component:
(component
(type $R1 (resource (rep i32)))
(type $R2 (resource (rep i32)))
(func $f1 (result (own $R1)) (canon lift ...))
(func $f2 (param (own $R2)) (canon lift ...))
)the types $R1 and $R2 are unequal and thus the return type of $f1
is incompatible with the parameter type of $f2.
The generativity of resource type definitions matches the abstract typing rules of type exports mentioned above, which force all clients of the component to bind a fresh abstract type. For example, in the following component:
(component
(component $C
(type $r1 (export "r1") (resource (rep i32)))
(type $r2 (export "r2") (resource (rep i32)))
)
(instance $c1 (instantiate $C))
(instance $c2 (instantiate $C))
(type $c1r1 (alias export $c1 "r1"))
(type $c1r2 (alias export $c1 "r2"))
(type $c2r1 (alias export $c2 "r1"))
(type $c2r2 (alias export $c2 "r2"))
)all four types aliases in the outer component are unequal, reflecting the fact
that each instance of $C generates two fresh resource types.
If a single resource type definition is exported more than once, the exports after the first are equality-bound to the first export. For example, the following component:
(component
(type $r (resource (rep i32)))
(export "r1" (type $r))
(export "r2" (type $r))
)is assigned the following componenttype:
(component
(export "r1" (type $r1 (sub resource)))
(export "r2" (type (eq $r1)))
)Thus, from an external perspective, r1 and r2 are two labels for the same
type.
If a component wants to hide this fact and force clients to assume r1 and
r2 are distinct types (thereby allowing the implementation to actually use
separate types in the future without breaking clients), an explicit type can be
ascribed to the export that replaces the eq bound with a less-precise sub
bound (using syntax introduced below).
(component
(type $r (resource (rep i32)))
(export "r1" (type $r))
(export "r2" (type $r) (type (sub resource)))
)This component is assigned the following componenttype:
(component
(export "r1" (type (sub resource)))
(export "r2" (type (sub resource)))
)The assignment of this type to the above component mirrors the introduction rule of existential types (∃T).
When supplying a resource type (imported or defined) to a type import via
instantiate, type checking performs a substitution, replacing all uses of the
import in the instantiated component with the actual type supplied via
with. For example, the following component validates:
(component $P
(import "C1" (component $C1
(import "T" (type $T (sub resource)))
(export "foo" (func (param (own $T))))
))
(import "C2" (component $C2
(import "T" (type $T (sub resource)))
(import "foo" (func (param (own $T))))
))
(type $R (resource (rep i32)))
(instance $c1 (instantiate $C1 (with "T" (type $R))))
(alias export $c1 "foo" (func $foo))
(instance $c2 (instantiate $C2 (with "T" (type $R)) (with "foo" (func $foo))))
)This depends critically on the T imports of $C1 and $C2 having been
replaced by $R when validating the instantiations of $c1 and $c2. These
type-checking rules for instantiating type imports mirror the elimination
rule of universal types (∀T).
Importantly, this type substitution performed by the parent is not visible to the child at validation- or run-time. In particular, there are no runtime casts that can "see through" to the original type parameter, avoiding avoiding the usual type-exposure problems with dynamic casts.
In summary: all type constructors are structural with the exception of
resource, which is abstract and generative. Type imports and exports that
have a subtype bound also introduce abstract types and follow the standard
introduction and elimination rules of universal and existential types.
Lastly, since "nominal" is often taken to mean "the opposite of structural", a
valid question is whether any of the above is "nominal typing". Inside a
component, resource types act "nominally": each resource type definition
produces a new local "name" for a resource type that is distinct from all
preceding resource types. The interesting case is when resource type equality
is considered from outside the component, particularly when a single
component is instantiated multiple times. In this case, a single resource type
definition that is exported with a single exportname will get a fresh type
with each component instance, with the abstract typing rules mentioned above
ensuring that each of the component's instance's resource types are kept
distinct. Thus, in a sense, the generativity of resource types generalizes
traditional name-based nominal typing, providing a finer granularity of
isolation than otherwise achievable with a shared global namespace.
From the perspective of Core WebAssembly running inside a component, the
Component Model is an embedder. As such, the Component Model defines the
Core WebAssembly imports passed to module_instantiate and how Core
WebAssembly exports are called via func_invoke. This allows the Component
Model to specify how core modules are linked together (as shown above) but it
also allows the Component Model to arbitrarily synthesize Core WebAssembly
functions (via func_alloc) that are imported by Core WebAssembly. These
synthetic core functions are created via one of several canonical definitions
defined below.
To implement or call a component-level function, we need to cross a
shared-nothing boundary. Traditionally, this problem is solved by defining a
serialization format. The Component Model MVP uses roughly this same approach,
defining a linear-memory-based ABI called the "Canonical ABI" which
specifies, for any functype, a corresponding
core:functype and rules for copying
values into and out of linear memory. The Component Model differs from
traditional approaches, though, in that the ABI is configurable, allowing
multiple different memory representations of the same abstract value. In the
MVP, this configurability is limited to the small set of canonopt shown
below. However, Post-MVP, adapter functions could be added to allow far more
programmatic control.
The Canonical ABI is explicitly applied to "wrap" existing functions in one of two directions:
liftwraps a core function (of typecore:functype) to produce a component function (of typefunctype) that can be passed to other components.lowerwraps a component function (of typefunctype) to produce a core function (of typecore:functype) that can be imported and called from Core WebAssembly code inside the current component.
Canonical definitions specify one of these two wrapping directions, the function to wrap and a list of configuration options:
canon ::= (canon lift core-prefix(<core:funcidx>) <canonopt>* bind-id(<externdesc>))
| (canon lower <funcidx> <canonopt>* (core func <id>?))
canonopt ::= string-encoding=utf8
| string-encoding=utf16
| string-encoding=latin1+utf16
| (memory <core:memidx>)
| (realloc <core:funcidx>)
| (post-return <core:funcidx>)While the production externdesc accepts any sort, the validation rules
for canon lift would only allow the func sort. In the future, other sorts
may be added (viz., types), hence the explicit sort.
The string-encoding option specifies the encoding the Canonical ABI will use
for the string type. The latin1+utf16 encoding captures a common string
encoding across Java, JavaScript and .NET VMs and allows a dynamic choice
between either Latin-1 (which has a fixed 1-byte encoding, but limited Code
Point range) or UTF-16 (which can express all Code Points, but uses either
2 or 4 bytes per Code Point). If no string-encoding option is specified, the
default is UTF-8. It is a validation error to include more than one
string-encoding option.
The (memory ...) option specifies the memory that the Canonical ABI will
use to load and store values. If the Canonical ABI needs to load or store,
validation requires this option to be present (there is no default).
The (realloc ...) option specifies a core function that is validated to
have the following core function type:
(func (param $originalPtr i32)
(param $originalSize i32)
(param $alignment i32)
(param $newSize i32)
(result i32))The Canonical ABI will use realloc both to allocate (passing 0 for the
first two parameters) and reallocate. If the Canonical ABI needs realloc,
validation requires this option to be present (there is no default).
The (post-return ...) option may only be present in canon lift
and specifies a core function to be called with the original return values
after they have finished being read, allowing memory to be deallocated and
destructors called. This immediate is always optional but, if present, is
validated to have parameters matching the callee's return type and empty
results.
Based on this description of the AST, the Canonical ABI explainer gives a detailed walkthrough of the static and dynamic semantics of lift
and lower.
One high-level consequence of the dynamic semantics of canon lift given in
the Canonical ABI explainer is that component functions are different from core
functions in that all control flow transfer is explicitly reflected in their
type. For example, with Core WebAssembly exception-handling and
stack-switching, a core function with type (func (result i32)) can return
an i32, throw, suspend or trap. In contrast, a component function with type
(func (result string)) may only return a string or trap. To express
failure, component functions can return result and languages with exception
handling can bind exceptions to the error case. Similarly, the forthcoming
addition of future and stream types would explicitly declare patterns of
stack-switching in component function signatures.
Similar to the import and alias abbreviations shown above, canon
definitions can also be written in an inverted form that puts the sort first:
(func $f (import "i" "f") ...type...) ≡ (import "i" "f" (func $f ...type...)) (WebAssembly 1.0)
(func $g ...type... (canon lift ...)) ≡ (canon lift ... (func $g ...type...))
(core func $h (canon lower ...)) ≡ (canon lower ... (core func $h))Note: in the future, canon may be generalized to define other sorts than
functions (such as types), hence the explicit sort.
Using canonical function definitions, we can finally write a non-trivial component that takes a string, does some logging, then returns a string.
(component
(import "logging" (instance $logging
(export "log" (func (param string)))
))
(import "libc" (core module $Libc
(export "mem" (memory 1))
(export "realloc" (func (param i32 i32) (result i32)))
))
(core instance $libc (instantiate $Libc))
(core func $log (canon lower
(func $logging "log")
(memory (core memory $libc "mem")) (realloc (func $libc "realloc"))
))
(core module $Main
(import "libc" "memory" (memory 1))
(import "libc" "realloc" (func (param i32 i32) (result i32)))
(import "logging" "log" (func $log (param i32 i32)))
(func (export "run") (param i32 i32) (result i32)
... (call $log) ...
)
)
(core instance $main (instantiate $Main
(with "libc" (instance $libc))
(with "logging" (instance (export "log" (func $log))))
))
(func $run (param string) (result string) (canon lift
(core func $main "run")
(memory (core memory $libc "mem")) (realloc (func $libc "realloc"))
))
(export "run" (func $run))
)This example shows the pattern of splitting out a reusable language runtime
module ($Libc) from a component-specific, non-reusable module ($Main). In
addition to reducing code size and increasing code-sharing in multi-component
scenarios, this separation allows $libc to be created first, so that its
exports are available for reference by canon lower. Without this separation
(if $Main contained the memory and allocation functions), there would be a
cyclic dependency between canon lower and $Main that would have to be
broken using an auxiliary module performing call_indirect.
In addition to the lift and lower canonical function definitions which
adapt existing functions, there are also a set of canonical "built-ins" that
define core functions out of nothing that can be imported by core modules to
dynamically interact with Canonical ABI entities like resources (and, when
async is added to the proposal, tasks).
canon ::= ...
| (canon resource.new <typeidx> (core func <id>?))
| (canon resource.drop <typeidx> (core func <id>?))
| (canon resource.rep <typeidx> (core func <id>?))
| (canon thread.spawn <typeidx> (core func <id>?)) 🧵
| (canon thread.hw_concurrency (core func <id>?)) 🧵The resource.new built-in has type [i32] -> [i32] and creates a new
resource (with resource type typeidx) with the given i32 value as its
representation and returning the i32 index of a new handle pointing to this
resource.
The resource.drop built-in has type [i32] -> [] and drops a resource handle
(with resource type typeidx) at the given i32 index. If the dropped handle
owns the resource, the resource's dtor is called, if present.
The resource.rep built-in has type [i32] -> [i32] and returns the i32
representation of the resource (with resource type typeidx) pointed to by the
handle at the given i32 index.
As an example, the following component imports the resource.new built-in,
allowing it to create and return new resources to its client:
(component
(import "Libc" (core module $Libc ...))
(core instance $libc (instantiate $Libc))
(type $R (resource (rep i32) (dtor (func $libc "free"))))
(core func $R_new (param i32) (result i32)
(canon resource.new $R)
)
(core module $Main
(import "canon" "R_new" (func $R_new (param i32) (result i32)))
(func (export "make_R") (param ...) (result i32)
(return (call $R_new ...))
)
)
(core instance $main (instantiate $Main
(with "canon" (instance (export "R_new" (func $R_new))))
))
(export $R' "r" (type $R))
(func (export "make-r") (param ...) (result (own $R'))
(canon lift (core func $main "make_R"))
)
)Here, the i32 returned by resource.new, which is an index into the
component's handle-table, is immediately returned by make_R, thereby
transferring ownership of the newly-created resource to the export's caller.
The shared-everything-threads proposal adds component model built-ins for thread management. These are specified as built-ins and not core WebAssembly instructions because browsers expect this functionality to come from existing Web/JS APIs.
The thread.spawn built-in has type [f:(ref null $f) c:i32] -> [i32] and
spawns a new thread by invoking the shared function f while passing c to it,
returning whether a thread was successfully spawned.
The resource.hw_concurrency built-in has type [] -> [i32] and returns the
number of threads that can be expected to execute concurrently.
See the CanonicalABI.md for detailed definitions of each of these built-ins and their interactions.
Value definitions (in the value index space) are like immutable global definitions
in Core WebAssembly except that validation requires them to be consumed exactly
once at instantiation-time (i.e., they are linear).
Components may define values in the value index space using following syntax:
value ::= (value <id>? <valtype> <val>)
val ::= false | true
| <core:i64>
| <f64canon>
| nan
| '<core:stringchar>'
| <core:name>
| (record <val>+)
| (variant "<label>" <val>?)
| (list <val>*)
| (tuple <val>+)
| (flags "<label>"*)
| (enum "<label>")
| none | (some <val>)
| ok | (ok <val>) | error | (error <val>)
| (binary <core:datastring>)
f64canon ::= <core:f64> without the `nan:0x` case.The validation rules for value require the val to match the valtype.
The (binary ...) expression form provides an alternative syntax allowing the binary contents
of the value definition to be written directly in the text format, analogous to data segments,
avoiding the need to understand type information when encoding or decoding.
For example:
(component
(value $a bool true)
(value $b u8 1)
(value $c u16 2)
(value $d u32 3)
(value $e u64 4)
(value $f s8 5)
(value $g s16 6)
(value $h s32 7)
(value $i s64 8)
(value $j f32 9.1)
(value $k f64 9.2)
(value $l char 'a')
(value $m string "hello")
(value $n (record (field "a" bool) (field "b" u8)) (record true 1))
(value $o (variant (case "a" bool) (case "b" u8)) (variant "b" 1))
(value $p (list (result (option u8)))
(list
error
(ok (some 1))
(ok none)
error
(ok (some 2))
)
)
(value $q (tuple u8 u16 u32) (tuple 1 2 3))
(type $abc (flags "a" "b" "c"))
(value $r $abc (flags "a" "c"))
(value $s (enum "a" "b" "c") (enum "b"))
(value $t bool (binary "\00"))
(value $u string (binary "\07example"))
(type $complex
(tuple
(record
(field "a" (option string))
(field "b" (tuple (option u8) string))
)
(list char)
$abc
string
)
)
(value $complex1 (type $complex)
(tuple
(record
none
(tuple none "empty")
)
(list)
(flags)
""
)
)
(value $complex2 (type $complex)
(tuple
(record
(some "example")
(tuple (some 42) "hello")
)
(list 'a' 'b' 'c')
(flags "b" "a")
"hi"
)
)
)As with all definition sorts, values may be imported and exported by components. As an example value import:
(import "env" (value $env (record (field "locale" (option string)))))As this example suggests, value imports can serve as generalized environment
variables, allowing not just string, but the full range of valtype.
Values can also be exported. For example:
(component
(import "system-port" (value $port u16))
(value $url string "https://example.com")
(export "default-url" (value $url))
(export "default-port" (value $port))
)The inferred type of this component is:
(component
(import "system-port" (value $port u16))
(value $url string "https://example.com")
(export "default-url" (value (eq $url)))
(export "default-port" (value (eq $port)))
)Thus, by default, the precise constant or import being exported is propagated into the component's type and thus its public interface. In this way, value exports can act as semantic configuration data provided by the component to the host or other client tooling. Components can also keep the exact value being exported abstract (so that the precise value is not part of the type and public interface) using the "type ascription" feature mentioned in the imports and exports section below.
Like modules, components can have start functions that are called during
instantiation. Unlike modules, components can call start functions at multiple
points during instantiation with each such call having parameters and results.
Thus, start definitions in components look like function calls:
start ::= (start <funcidx> (value <valueidx>)* (result (value <id>?))*)The (value <valueidx>)* list specifies the arguments passed to funcidx by
indexing into the value index space. The arity and types of the two value lists are
validated to match the signature of funcidx.
With this, we can define a component that imports a string and computes a new exported string at instantiation time:
(component
(import "name" (value $name string))
(import "libc" (core module $Libc
(export "memory" (memory 1))
(export "realloc" (func (param i32 i32 i32 i32) (result i32)))
))
(core instance $libc (instantiate $Libc))
(core module $Main
(import "libc" ...)
(func (export "start") (param i32 i32) (result i32)
... general-purpose compute
)
)
(core instance $main (instantiate $Main (with "libc" (instance $libc))))
(func $start (param string) (result string) (canon lift
(core func $main "start")
(memory (core memory $libc "mem")) (realloc (func $libc "realloc"))
))
(start $start (value $name) (result (value $greeting)))
(export "greeting" (value $greeting))
)As this example shows, start functions reuse the same Canonical ABI machinery as normal imports and exports for getting component-level values into and out of core linear memory.
Both import and export definitions append a new element to the index space of
the imported/exported sort which can be optionally bound to an identifier in
the text format. In the case of imports, the identifier is bound just like Core
WebAssembly, as part of the externdesc (e.g., (import "x" (func $x)) binds
the identifier $x). In the case of exports, the <id>? right after the
export is bound while the <id> inside the <sortidx> is a reference to the
preceding definition being exported (e.g., (export $x "x" (func $f)) binds a
new identifier $x).
import ::= (import "<importname>" bind-id(<externdesc>))
export ::= (export <id>? "<exportname>" <sortidx> <externdesc>?)All import names are required to be unique and all export names are required to be unique. The rest of the grammar for imports and exports defines a structured syntax for the contents of import and export names. Syntactically, these names appear inside quoted string literals. The grammar thus restricts the contents of these string literals to provide more structured information that can be mechanically interpreted by toolchains and runtimes to support idiomatic developer workflows and source-language bindings. The rules defining this structured name syntax below are to be interpreted as a lexical grammar defining a single token and thus whitespace is not automatically inserted, all terminals are single-quoted, and everything unquoted is a meta-character.
exportname ::= <plainname>
| <interfacename>
importname ::= <exportname>
| <depname>
| <urlname>
| <hashname>
plainname ::= <label>
| '[constructor]' <label>
| '[method]' <label> '.' <label>
| '[static]' <label> '.' <label>
label ::= <fragment>
| <label> '-' <fragment>
fragment ::= <word>
| <acronym>
word ::= [a-z] [0-9a-z]*
acronym ::= [A-Z] [0-9A-Z]*
interfacename ::= <namespace> <label> <projection> <version>?
| <namespace>+ <label> <projection>+ <version>? 🪺
namespace ::= <words> ':'
words ::= <word>
| <words> '-' <word>
projection ::= '/' <label>
version ::= '@' <valid semver>
depname ::= 'unlocked-dep=<' <pkgnamequery> '>'
| 'locked-dep=<' <pkgname> '>' ( ',' <hashname> )?
pkgnamequery ::= <pkgpath> <verrange>?
pkgname ::= <pkgpath> <version>?
pkgpath ::= <namespace> <words>
| <namespace>+ <words> <projection>* 🪺
verrange ::= '@*'
| '@{' <verlower> '}'
| '@{' <verupper> '}'
| '@{' <verlower> ' ' <verupper> '}'
verlower ::= '>=' <valid semver>
verupper ::= '<' <valid semver>
urlname ::= 'url=<' <nonbrackets> '>' (',' <hashname>)?
nonbrackets ::= [^<>]*
hashname ::= 'integrity=<' <integrity-metadata> '>'Components provide six options for naming imports:
- a plain name that leaves it up to the developer to "read the docs" or otherwise figure out what to supply for the import;
- an interface name that is assumed to uniquely identify a higher-level semantic contract that the component is requesting an unspecified wasm or native implementation of;
- a URL name that the component is requesting be resolved to a particular wasm implementation by fetching the URL.
- a hash name containing a content-hash of the bytes of a particular wasm implemenentation but not specifying location of the bytes.
- a locked dependency name that the component is requesting be resolved via some contextually-supplied registry to a particular wasm implementation using the given hierarchical name and version; and
- an unlocked dependency name that the component is requesting be resolved via some contextually-supplied registry to one of a set of possible of wasm implementations using the given hierarchical name and version range.
Not all hosts are expected to support all six import naming options and, in general, build tools may need to wrap a to-be-deployed component with an outer component that only uses import names that are understood by the target host. For example:
- an offline host may only implement a fixed set of interface names, requiring a build tool to bundle URL, dependency and hash names (replacing the imports with nested definitions);
- browsers may only support plain and URL names (with plain names resolved via import map or JS API), requiring the build process to publish or bundle dependencies, converting dependency names into nested definitions or URL names;
- a production server environment may only allow deployment of components importing from a fixed set of interface and locked dependency names, thereby requiring all dependencies to be locked and deployed beforehand;
- host embeddings without a direct developer interface (such as the JS API or import maps) may reject all plain names, requiring the build process to resolve these beforehand;
- hosts without content-addressable storage may reject hash names (as they have no way to locate the contents).
The grammar and validation of URL names allows the embedded URLs to contain any sequence of UTF-8 characters (other than angle brackets, which are used to delimit the URL), leaving the well-formedness of the URL to be checked as part of the process of parsing the URL in preparation for fetching the URL. The base URL operand passed to the URL spec's parsing algorithm is determined by the host and may be absent, thereby disallowing relative URLs. Thus, the parsing and fetching of a URL import are host-defined operations that happen after the decoding and validation of a component, but before instantiation of that component.
When a particular implementation is indicated via URL or dependency name,
importname allows the component to additionally specify a cryptographic hash
of the expected binary representation of the wasm implementation, reusing the
integrity-metadata production defined by the W3C Subresource Integrity
specification. When this hash is present, a component can express its intention
to reuse another component or core module with the same degree of specificity
as if the component or core module was nested directly, thereby allowing
components to factor out common dependencies without compromising runtime
behavior. When only the hash is present (in a hashname), the host must
locate the contents using the hash (e.g., using an OCI Registry).
The "registry" referred to by dependency names serves to map a hierarchical
name and version to a particular module, component or exported definition. For
example, in the full generality of nested namespaces and packages (🪺), in a
registry name a:b:c/d/e/f, a:b:c traverses a path through namespaces a
and b to a component c and /d/e/f traverses the exports of c (where d
and e must be component exports but f can be anything). Given this abstract
definition, a number of concrete data sources can be interpreted by developer
tooling as "registries":
- a live registry (perhaps accessed via
warg) - a local filesystem directory (perhaps containing vendored dependencies)
- a fixed set of host-provided functionality (see also the built-in modules proposal)
- a programmatically-created tree data structure (such as the
importObjectparameter ofWebAssembly.instantiate())
The valid semver production is as defined by the Semantic Versioning 2.0
spec and is meant to be interpreted according to that specification. The
verrange production embeds a minimal subset of the syntax for version ranges
found in common package managers like npm and cargo and is meant to be
interpreted with the same semantics. (Mostly this
interpretation is the usual SemVer-spec-defined ordering, but note the
particular behavior of pre-release tags.)
The plainname production captures several language-neutral syntactic hints
that allow bindings generators to produce more idiomatic bindings in their
target language. At the top-level, a plainname allows functions to be
annotated as being a constructor, method or static function of a preceding
resource. In each of these cases, the first label is the name of the resource
and the second label is the logical field name of the function. This
additional nesting information allows bindings generators to insert the
function into the nested scope of a class, abstract data type, object,
namespace, package, module or whatever resources get bound to. For example, a
function named [method]C.foo could be bound in C++ to a member function foo
in a class C. The JS API below describes how the native JavaScript
bindings could look. Validation described in Binary.md inspects
the contents of plainname and ensures that the function has a compatible
signature.
The label production used inside plainname as well as the labels of
record and variant types are required to have kebab case. The reason for
this particular form of casing is to unambiguously separate words and acronyms
(represented as all-caps words) so that source language bindings can convert a
label into the idiomatic casing of that language. (Indeed, because hyphens
are often invalid in identifiers, kebab case practically forces language
bindings to make such a conversion.) For example, the label is-XML could be
mapped to isXML, IsXml, is_XML or is_xml, depending on the target
language/convention. The highly-restricted character set ensures that
capitalization is trivial and does not require consulting Unicode tables.
Because some casing schemes (such as all-lowercase) would lead to clashes if
two labels differed only in case, in all cases where "uniquness" is required
between a set of names (viz., import/export names, record field labels, variant
case labels, and function parameter/result names), two labels that differ
only in case are considered equal and thus rejected.
Components provide two options for naming exports, symmetric to the first two options for naming imports:
- a plain name that leaves it up to the developer to "read the docs" or otherwise figure out what the export does and how to use it; and
- an interface name that is assumed to uniquely identify a higher-level semantic contract that the component is claiming to implement with the given exported definition.
As an example, the following component uses all 9 cases of imports and exports:
(component
(import "custom-hook" (func (param string) (result string)))
(import "wasi:http/handler" (instance
(export "request" (type $request (sub resource)))
(export "response" (type $response (sub resource)))
(export "handle" (func (param (own $request)) (result (own $response))))
))
(import "url=<https://mycdn.com/my-component.wasm>" (component ...))
(import "url=<./other-component.wasm>,integrity=<sha256-X9ArH3k...>" (component ...))
(import "locked-dep=<my-registry:[email protected]>,integrity=<sha256-H8BRh8j...>" (component ...))
(import "unlocked-dep=<my-registry:imagemagick@{>=1.0.0}>" (instance ...))
(import "integrity=<sha256-Y3BsI4l...>" (component ...))
... impl
(export "wasi:http/handler" (instance $http_handler_impl))
(export "get-JSON" (func $get_json_impl))
)Here, custom-hook and get-JSON are plain names for functions whose semantic
contract is particular to this component and not defined elsewhere. In
contrast, wasi:http/handler is the name of a separately-defined interface,
allowing the component to request the ability to make outgoing HTTP requests
(through imports) and receive incoming HTTP requests (through exports) in a way
that can be mechanically interpreted by hosts and tooling.
The remaining 4 imports show the different ways that a component can import
external implementations. Here, the URL and locked dependency imports use
component types, allowing this component to privately create and wire up
instances using instance definitions. In contrast, the unlocked dependency
import uses an instance type, anticipating a subsequent tooling step (likely
the one that performs dependency resolution) to select, instantiate and provide
the instance.
Validation of export requires that all transitive uses of resource types in
the types of exported functions or values refer to resources that were either
imported or exported (concretely, via the type index introduced by an import
or export). The optional <externdesc>? in export can be used to
explicitly ascribe a type to an export which is validated to be a supertype of
the definition's type, thereby allowing a private (non-exported) type
definition to be replaced with a public (exported) type definition.
For example, in the following component:
(component
(import "R1" (type $R1 (sub resource)))
(type $R2 (resource (rep i32)))
(export $R2' "R2" (type $R2))
(func $f1 (result (own $R1)) (canon lift ...))
(func $f2 (result (own $R2)) (canon lift ...))
(func $f2' (result (own $R2')) (canon lift ...))
(export "f1" (func $f1))
;; (export "f2" (func $f2)) -- invalid
(export "f2" (func $f2) (func (result (own $R2'))))
(export "f2" (func $f2'))
)the commented-out export is invalid because its type transitively refers to
$R2, which is a private type definition. This requirement is meant to address
the standard avoidance problem that appears in module systems with abstract
types. In particular, it ensures that a client of a component is able to
externally define a type compatible with the exports of the component.
Similar to type exports, value exports may also ascribe a type to keep the precise value from becoming part of the type and public interface.
For example:
(component
(value $url string "https://example.com")
(export "default-url" (value $url) (value string))
)The inferred type of this component is:
(component
(export "default-url" (value string))
)Note, that the url value definition is absent from the component type
As a consequence of the shared-nothing design described above, all calls into or out of a component instance necessarily transit through a component function definition. Thus, component functions form a "membrane" around the collection of core module instances contained by a component instance, allowing the Component Model to establish invariants that increase optimizability and composability in ways not otherwise possible in the shared-everything setting of Core WebAssembly. The Component Model proposes establishing the following three runtime invariants:
- Components define a "lockdown" state that prevents continued execution after a trap. This both prevents continued execution with corrupt state and also allows more-aggressive compiler optimizations (e.g., store reordering). This was considered early in Core WebAssembly standardization but rejected due to the lack of clear trapping boundary. With components, each component instance is given a mutable "lockdown" state that is set upon trap and implicitly checked at every execution step by component functions. Thus, after a trap, it's no longer possible to observe the internal state of a component instance.
- Components prevent unexpected reentrance by setting the "lockdown" state (in the previous bullet) whenever calling out through an import, clearing the lockdown state on return, thereby preventing reentrant export calls in the interim. This establishes a clear contract between separate components that both prevents obscure composition-time bugs and also enables more-efficient non-reentrant runtime glue code (particularly in the middle of the Canonical ABI). This implies that components by default don't allow concurrency and multi-threaded access will trap.
The JS API currently provides WebAssembly.compile(Streaming) which take
raw bytes from an ArrayBuffer or Response object and produces
WebAssembly.Module objects that represent decoded and validated modules. To
natively support the Component Model, the JS API would be extended to allow
these same JS API functions to accept component binaries and produce new
WebAssembly.Component objects that represent decoded and validated
components. The binary format of components is designed to allow
modules and components to be distinguished by the first 8 bytes of the binary
(splitting the 32-bit core:version field into a 16-bit version field and
a 16-bit layer field with 0 for modules and 1 for components).
Once compiled, a WebAssembly.Component could be instantiated using the
existing JS API WebAssembly.instantiate(Streaming). Since components have the
same basic import/export structure as modules, this means extending the read
the imports logic to support single-level imports as well as imports of
modules, components and instances. Since the results of instantiating a
component is a record of JavaScript values, just like an instantiated module,
WebAssembly.instantiate would always produce a WebAssembly.Instance object
for both module and component arguments.
Types are a new sort of definition that are not (yet) present in Core WebAssembly and so the read the imports and create an exports object steps need to be expanded to cover them:
For type exports, each type definition would export a JS constructor function.
This function would be callable iff a [constructor]-annotated function was
also exported. All [method]- and [static]-annotated functions would be
dynamically installed on the constructor's prototype chain. In the case of
re-exports and multiple exports of the same definition, the same constructor
function object would be exported (following the same rules as WebAssembly
Exported Functions today). In pathological cases (which, importantly, don't
concern the global namespace, but involve the same actual type definition being
imported and re-exported by multiple components), there can be collisions when
installing constructors, methods and statics on the same constructor function
object. In such cases, a conservative option is to undo the initial
installation and require all clients to instead use the full explicit names
as normal instance exports.
For type imports, the constructors created by type exports would naturally
be importable. Additionally, certain JS- and Web-defined objects that correspond
to types (e.g., the RegExp and ArrayBuffer constructors or any Web IDL
interface object) could be imported. The ToWebAssemblyValue checks on
handle values mentioned below can then be defined to perform the associated
internal slot type test, thereby providing static type guarantees for
outgoing handles that can avoid runtime dynamic type tests.
Lastly, when given a component binary, the compile-then-instantiate overloads
of WebAssembly.instantiate(Streaming) would inherit the compound behavior of
the abovementioned functions (again, using the layer field to eagerly
distinguish between modules and components).
For example, the following component:
;; a.wasm
(component
(import "one" (func))
(import "two" (value string)) 🪙
(import "three" (instance
(export "four" (instance
(export "five" (core module
(import "six" "a" (func))
(import "six" "b" (func))
))
))
))
...
)and module:
;; b.wasm
(module
(import "six" "a" (func))
(import "six" "b" (func))
...
)could be successfully instantiated via:
WebAssembly.instantiateStreaming(fetch('./a.wasm'), {
one: () => (),
two: "hi", 🪙
three: {
four: {
five: await WebAssembly.compileStreaming(fetch('./b.wasm'))
}
}
});The other significant addition to the JS API would be the expansion of the set
of WebAssembly types coerced to and from JavaScript values (by ToJSValue
and ToWebAssemblyValue) to include all of valtype.
At a high level, the additional coercions would be:
| Type | ToJSValue |
ToWebAssemblyValue |
|---|---|---|
bool |
true or false |
ToBoolean |
s8, s16, s32 |
as a Number value | ToInt8, ToInt16, ToInt32 |
u8, u16, u32 |
as a Number value | ToUint8, ToUint16, ToUint32 |
s64 |
as a BigInt value | ToBigInt64 |
u64 |
as a BigInt value | ToBigUint64 |
f32, f64 |
as a Number value | ToNumber |
char |
same as USVString |
same as USVString, throw if the USV length is not 1 |
record |
TBD: maybe a JS Record? | same as dictionary |
variant |
see below | see below |
list |
create a typed array copy for number types; otherwise produce a JS array (like sequence) |
same as sequence |
string |
same as USVString |
same as USVString |
tuple |
TBD: maybe a JS Tuple? | TBD |
flags |
TBD: maybe a JS Record? | same as dictionary of optional boolean fields with default values of false |
enum |
same as enum |
same as enum |
option |
same as T? |
same as T? |
result |
same as variant, but coerce a top-level error return value to a thrown exception |
same as variant, but coerce uncaught exceptions to top-level error return values |
own, borrow |
see below | see below |
Notes:
- Function parameter names are ignored since JavaScript doesn't have named parameters.
- If a function's result type list is empty, the JavaScript function returns
undefined. If the result type list contains a single unnamed result, then the return value is specified byToJSValueabove. Otherwise, the function result is wrapped into a JS object whose field names are taken from the result names and whose field values are specified byToJSValueabove. - In lieu of an existing standard JS representation for
variant, the JS API would need to define its own custom binding built from objects. As a sketch, the JS values accepted by(variant (case "a" u32) (case "b" string))could include{ tag: 'a', value: 42 }and{ tag: 'b', value: "hi" }. - For
option, when Web IDL doesn't support particular type combinations (e.g.,(option (option u32))), the JS API would fall back to the JS API of the unspecializedvariant(e.g.,(variant (case "some" (option u32)) (case "none")), despecializing only the problematic outeroption). - When coercing
ToWebAssemblyValue,ownandborrowhandle types would dynamically guard that the incoming JS value's dynamic type was compatible with the imported resource type referenced by the handle type. For example, if a component contains(import "Object" (type $Object (sub resource)))and is instantiated with the JSObjectconstructor, then(own $Object)and(borrow $Object)could accept JSobjectvalues. - When coercing
ToJSValue, handle values would be wrapped with JS objects that are instances of the handles' resource type's exported constructor (described above). Forownhandles, aFinalizationRegistrywould be used to drop theownhandle (thereby calling the resource destructor) when its wrapper object was unreachable from JS. Forborrowhandles, the wrapper object would become dynamically invalid (throwing on any access) at the end of the export call. - The forthcoming addition of future and stream types would allow
PromiseandReadableStreamvalues to be passed directly to and from components without requiring handles or callbacks. - When an imported JavaScript function is a built-in function wrapping a Web IDL function, the specified behavior should allow the intermediate JavaScript call to be optimized away when the types are sufficiently compatible, falling back to a plain call through JavaScript when the types are incompatible or when the engine does not provide a separate optimized call path.
Like the JS API, ESM-integration can be extended to load components in all
the same places where modules can be loaded today, branching on the layer
field in the binary format to determine whether to decode as a module or a
component.
For URL import names, the embedded URL would be used as the Module Specifier. For plain names, the whole plain name would be used as the Module Specifier (and an import map would be needed to map the string to a URL). For locked and unlocked dependency names, ESM-integration would likely simply fail loading the module, requiring a bundler to map these registry-relative names to URLs.
TODO: ESM-integration for interface imports and exports is still being worked out in detail.
The main remaining question is how to deal with component imports having a single string as well as the new importable component, module and instance types. Going through these one by one:
For component imports of module type, we need a new way to request that the ESM loader parse or decode a module without also instantiating that module. Recognizing this same need from JavaScript, there is a TC39 proposal called Import Reflection that adds the ability to write, in JavaScript:
import Foo from "./foo.wasm" as "wasm-module";
assert(Foo instanceof WebAssembly.Module);With this extension to JavaScript and the ESM loader, a component import
of module type can be treated the same as import ... as "wasm-module".
Component imports of component type would work the same way as modules,
potentially replacing "wasm-module" with "wasm-component".
In all other cases, the (single) string imported by a component is first
resolved to a Module Record using the same process as resolving the
Module Specifier of a JavaScript import. After this, the handling of the
imported Module Record is determined by the import type:
For imports of instance type, the ESM loader would treat the exports of the
instance type as if they were the Named Imports of a JavaScript import.
Thus, single-level imports of instance type act like the two-level imports
of Core WebAssembly modules where the first-level has been factored out. Since
the exports of an instance type can themselves be instance types, this process
must be performed recursively.
Otherwise, function or value imports are treated like an Imported Default Binding and the Module Record is converted to its default value. This allows the following component:
;; bar.wasm
(component
(import "./foo.js" (func (result string)))
...
)to be satisfied by a JavaScript module via ESM-integration:
// foo.js
export default () => "hi";when bar.wasm is loaded as an ESM:
<script src="bar.wasm" type="module"></script>For some use-case-focused, worked examples, see:
- Link-time virtualization example
- Shared-everything dynamic linking example
- Component Examples presentation
The following features are needed to address the MVP Use Cases and will be added over the coming months to complete the MVP proposal:
- concurrency support (slides)
- optional imports, definitions and exports (subsuming WASI Optional Imports and maybe conditional-sections)