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Monadic Contexts
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Monadic Contexts

[!WARNING] Reword for people without Haskell background who don't know what lifting is.

Coming from a Haskell background, we have found that Monads provide a great abstraction with which to reason about program behaviour, but they have some severe usability issues. The main one of these is the lack of automatic lifting, requiring users to explicitly lift computations through their monad transformer stack.

For a language as focused on usability as Enso is importing all the complexity of Haskell monads really isn't feasible. To that end, we have created the notion of a 'Monadic Context', which is a monad transformer based on Supermonads (see references). These have special support in the compiler, and hence can be automatically lifted to aid usability.

The actionables for this section are:

  • Think about subsumption for contexts.
  • Contexts (e.g. IO) are represented using T in IO. Multiple contexts are combined as standard (IO | State Int), and it is written the same in arg position.
  • Do we definitely want to use monads, or can we use arrows or other interpreter-based effects systems? These may aid with parallelism analysis.

Context Syntax

[!WARNING] There used to be three main notes about the syntax of contexts:

  1. Monadic contexts are defined using the in keyword (e.g. Int in IO).
  2. We have a symbol !, which is short-hand for putting something into the Exception monadic context. This is related to broken values.
  3. Contexts can be combined by using the standard typeset operators, or nested through repeated uses of in.

There is no special syntax for contexts anymore. Since #3828 Enso is no longer relaying on a haskelly solution. Rather than that contexts are being manupulated by standard library functions grouped around Standard.Base.Runtime.Context & co.

Runtime.Context.Output.with_enabled <|
    File.new "c:\trash.txt" . delete

There is still the ! symbol signaling presence of errors

  • e.g. broken values. However the runtime can handle broken values even without presence of these exception type signatures. Thus the compiler only verifies the referenced types are valid.

Monadic Bind

[!WARNING] Who knows what <- means in Haskell?

It is also important to note that Enso has no equivalent to <- in Haskell. Instead, pure computations are implicitly placed in the Pure monadic context, and = acts to 'peel off' the outermost layer of contexts. As such, this means that = always acts as bind, greatly simplifying how the type-checker has to work.

Inbuilt Contexts

Enso standard library defines Input, Output and Dataflow_Stack_Trace contects as of Enso 2024.5.1 version. Users cannot define their own.

State

The state concept is implement by standard libraries with no support in the type system.

State acts as a thread local variable of operating system:

  • an initializing code can set State up
  • execute some code
  • a code somewhere deep the stack (while initializing code is still on the stack)
  • may pick the state up
  • once the initializing code finishes execution
  • the state is gone

It is an example of tunnelling a value from one side (e.g. code) of the "tunnel" to another, without the "tunnel" (e.g. thee code in between) knowing about it.

See Standard.Base.Runtime.State for more details.