docs: update getp, setp docstring, add tutorial

docs/pages.jl

@@ -3,4 +3,5 @@
 pages = [
     "Home" => "index.md",
     "API" => "api.md",
+    "Tutorial" => "tutorial.md",
 ]

docs/src/tutorial.md

@@ -0,0 +1,193 @@
+# Implementing SymbolicIndexingInterface for a type
+
+Implementing the interface for a type allows it to be used by existing symbolic indexing
+infrastructure. There are multiple ways to implement it, and the entire interface is
+not always necessary.
+
+## Defining a fallback
+
+The simplest case is when the type contains an object that already implements the interface.
+All its methods can simply be forwarded to that object. To do so, SymbolicIndexingInterface.jl
+provides the [`symbolic_container`](@ref) method. For example,
+
+```julia
+struct MySolutionWrapper{T<:SciMLBase.AbstractTimeseriesSolution}
+  sol::T
+  # other properties...
+end
+
+symbolic_container(sys::MySolutionWrapper) = sys.sol
+```
+
+`MySolutionWrapper` wraps an `AbstractTimeseriesSolution` which already implements the interface.
+Since `symbolic_container` will return the wrapped solution, all method calls such as
+`is_parameter(sys::MySolutionWrapper, sym)` will be forwarded to `is_parameter(sys.sol, sym)`.
+
+In case some methods need to function differently than those of the wrapped type, they can selectively
+be defined. For example, suppose `MySolutionWrapper` does not support observed quantities. The following
+method can be defined (in addition to the one above):
+
+```julia
+is_observed(sys::MySolutionWrapper, sym) = false
+```
+
+## Defining the interface in its entirety
+
+Not all of the methods in the interface are required. Some only need to be implemented if a type
+supports specific functionality. Consider the following struct which needs to implement the interface:
+
+```julia
+struct ExampleSolution
+  state_index::Dict{Symbol,Int}
+  parameter_index::Dict{Symbol,Int}
+  independent_variable::Union{Symbol,Nothing}
+  u::Vector{Vector{Float64}}
+  p::Vector{Float64}
+  t::Vector{Float64}
+end
+```
+
+
+
+### Mandatory methods
+
+```julia
+function SymbolicIndexingInterface.is_variable(sys::ExampleSolution, sym)
+  haskey(sys.state_index, sym)
+end
+
+function SymbolicIndexingInterface.variable_index(sys::ExampleSolution, sym)
+  get(sys.state_index, sym, nothing)
+end
+
+function SymbolicIndexingInterface.variable_symbols(sys::ExampleSolution)
+  collect(keys(sys.state_index))
+end
+
+function SymbolicIndexingInterface.is_parameter(sys::ExampleSolution, sym)
+  haskey(sys.parameter_index, sym)
+end
+
+function SymbolicIndexingInterface.parameter_index(sys::ExampleSolution, sym)
+  get(sys.parameter_index, sym, nothing)
+end
+
+function SymbolicIndexingInterface.parameter_symbols(sys::ExampleSolution)
+  collect(keys(sys.parameter_index))
+end
+
+function SymbolicIndexingInterface.is_independent_variable(sys::ExampleSolution, sym)
+  # note we have to check separately for `nothing`, otherwise
+  # `is_independent_variable(p, nothing)` would return `true`.
+  sys.independent_variable !== nothing && sym === sys.independent_variable
+end
+
+function SymbolicIndexingInterface.independent_variable_symbols(sys::ExampleSolution)
+  sys.independent_variable === nothing ? [] : [sys.independent_variable]
+end
+
+# this types accepts `Expr` for observed expressions involving state/parameter
+# variables
+SymbolicIndexingInterface.is_observed(sys::ExampleSolution, sym) = sym isa Expr
+
+function SymbolicIndexingInterface.observed(sys::ExampleSolution, sym::Expr)
+  if is_time_dependent(sys)
+    return function (u, p, t)
+      # compute value from `sym`, leveraging `variable_index` and
+      # `parameter_index` to turn symbols into indices
+    end
+  else
+    return function (u, p)
+      # compute value from `sym`, leveraging `variable_index` and
+      # `parameter_index` to turn symbols into indices
+    end
+  end
+end
+
+function SymbolicIndexingInterface.is_time_dependent(sys::ExampleSolution)
+  sys.independent_variable !== nothing
+end
+
+SymbolicIndexingInterface.constant_structure(::ExampleSolution) = true
+```
+
+Note that the method definitions are all assuming `constant_structure(p) == true`.
+
+In case `constant_structure(p) == false`, the following methods would change:
+- `constant_structure(::ExampleSolution) = false`
+- `variable_index(sys::ExampleSolution, sym)` would become
+  `variable_index(sys::ExampleSolution, sym i)` where `i` is the time index at which
+  the index of `sym` is required.
+- `variable_symbols(sys::ExampleSolution)` would become
+  `variable_symbols(sys::ExampleSolution, i)` where `i` is the time index at which
+  the variable symbols are required.
+- `observed(sys::ExampleSolution, sym)` would become
+  `observed(sys::ExampleSolution, sym, i)` where `i` is either the time index at which
+  the index of `sym` is required or a `Vector` of state symbols at the current time index.
+
+## Optional methods
+
+Note that `observed` is optional if `is_observed` is always `false`, or the type is
+only responsible for identifying observed values and `observed` will always be called
+on a type that wraps this type. An example is `ModelingToolkit.AbstractSystem`, which
+can identify whether a value is observed, but cannot implement `observed` itself.
+
+Other optional methods relate to parameter indexing. If a type contains the values of
+parameter variables, it must implement [`parameter_values`](@ref). This will allow the
+default definitions of [`getp`](@ref) and [`setp`](@ref) to work. While `setp` is
+not typically useful for solution objects, it may be useful for integrators. Typically
+the default implementations for `getp` and `setp` will suffice and manually defining
+them is not necessary.
+
+```julia
+function SymbolicIndexingInterface.parameter_values(sys::ExampleSolution)
+  sys.p
+end
+```
+
+# Implementing the `SymbolicTypeTrait` for a type
+
+The `SymbolicTypeTrait` is used to identify values that can act as symbolic variables. It
+has three variants:
+- [`NotSymbolic`](@ref) for quantities that are not symbolic. This is the default for all
+  types.
+- [`ScalarSymbolic`](@ref) for quantities that are symbolic, and represent a single
+  logical value.
+- [`ArraySymbolic`](@ref) for quantities that are symbolic, and represent an array of
+  values. Types implementing this trait must return an array of `ScalarSymbolic` variables
+  of the appropriate size and dimensions when `collect`ed.
+
+The trait is implemented through the [`symbolic_type`](@ref) function. Consider the following
+example types:
+
+```julia
+struct MySym
+  name::Symbol
+end
+
+struct MySymArr{N}
+  name::Symbol
+  size::NTuple{N,Int}
+end
+```
+
+They must implement the following functions:
+
+```julia
+SymbolicIndexingInterface.symbolic_type(::Type{MySym}) = ScalarSymbolic()
+SymbolicIndexingInterface.hasname(::MySym) = true
+SymbolicIndexingInterface.getname(sym::MySym) = sym.name
+
+SymbolicIndexingInterface.symbolic_type(::Type{<:MySymArr}) = ArraySymbolic()
+SymbolicIndexingInterface.hasname(::MySymArr) = true
+SymbolicIndexingInterface.getname(sym::MySymArr) = sym.name
+function Base.collect(sym::MySymArr)
+  [
+    MySym(Symbol(sym.name, :_, join(idxs, "_")))
+    for idxs in Iterators.product(Base.OneTo.(sym.size)...)
+  ]
+end
+```
+
+[`hasname`](@ref) is not required to always be `true` for symbolic types. For example,
+`Symbolics.Num` returns `false` whenever the wrapped value is a number, or an expression.

src/parameter_indexing.jl

@@ -9,7 +9,9 @@ function parameter_values end
     getp(sys, p)
 
 Return a function that takes an integrator or solution of `sys`, and returns the value of
-the parameter `p`. Requires that the integrator or solution implement
+the parameter `p`. Note that `p` can be a direct numerical index or a symbolic value.
+Requires that the integrator or solution implement [`parameter_values`](@ref). This function
+typically does not need to be implemented, and has a default implementation relying on
 [`parameter_values`](@ref).
 """
 function getp(sys, p)
@@ -50,9 +52,11 @@ end
     setp(sys, p)
 
 Return a function that takes an integrator of `sys` and a value, and sets the
-the parameter `p` to that value. Requires that the integrator implement
-[`parameter_values`](@ref) and the returned collection be a mutable reference
-to the parameter vector in the integrator.
+the parameter `p` to that value. Note that `p` can be a direct numerical index or a
+symbolic value. Requires that the integrator implement [`parameter_values`](@ref) and the
+returned collection be a mutable reference to the parameter vector in the integrator. In
+case `parameter_values` cannot return such a mutable reference, `setp` needs to be
+implemented manually.
 """
 function setp(sys, p)
     symtype = symbolic_type(p)