ExpressionExplorer.jl

Find all variables referenced and defined in an expression. This package is used internally by Pluto to find links between cells.

Quick example

julia> using ExpressionExplorer

julia> ex = :(const words = split(line));

julia> node = ExpressionExplorer.compute_reactive_node(ex);

julia> node.references
Set{Symbol} with 2 elements:
  :line
  :split

julia> node.definitions
Set{Symbol} with 1 element:
  :words

API

The main function to use is compute_reactive_node(expression), which returns a ReactiveNode. There is also a more low-level API available: compute_symbols_state returning a SymbolsSate.

High-level: ReactiveNode

If you are interested in the dependencies between expressions, then you should compute the ReactiveNode for each expression. This is a data structure that looks like:

Base.@kwdef struct ReactiveNode
    # core fields:
    references::Set{Symbol} = Set{Symbol}()
    definitions::Set{Symbol} = Set{Symbol}()
    
    # more advanced fields:
    soft_definitions::Set{Symbol} = Set{Symbol}()
    funcdefs_with_signatures::Set{FunctionNameSignaturePair} = Set{FunctionNameSignaturePair}()
    funcdefs_without_signatures::Set{Symbol} = Set{Symbol}()
    macrocalls::Set{Symbol} = Set{Symbol}()
end

You can use the function compute_reactive_node(expression) to explore an expression and generate the resulting ReactiveNode.

Example for compute_reactive_node

Let's compute the ReactiveNode for these two expressions:

julia> e1 = :(weather = magic() + science);

julia> e2 = :(weather() = magic() + science);

First one:

julia> r1 = ExpressionExplorer.compute_reactive_node(e1)
ExpressionExplorer.ReactiveNode(Set([:+, :magic, :science]), Set([:weather]), Set{Symbol}(), Set{ExpressionExplorer.FunctionNameSignaturePair}(), Set{Symbol}(), Set{Symbol}())

julia> r1.definitions
Set{Symbol} with 1 element:
  :weather

julia> r1.references
Set{Symbol} with 3 elements:
  :+
  :magic
  :science

julia> r1.funcdefs_without_signatures
Set{Symbol}()

Second one, note that weather is a function definition, so it does not show up in r2.definitions. If you want everything that is defined, you can use r2.definitions ∪ r2.funcdefs_without_signatures.

julia> r2 = ExpressionExplorer.compute_reactive_node(e2)
ExpressionExplorer.ReactiveNode(Set([:+, :magic, :science]), Set{Symbol}(), Set{Symbol}(), Set(ExpressionExplorer.FunctionNameSignaturePair[ExpressionExplorer.FunctionNameSignaturePair([:weather], 0xa2e6e5b3d2eee6b5)]), Set([:weather]), Set{Symbol}())

julia> r2.definitions
Set{Symbol}()

julia> r2.references
Set{Symbol} with 3 elements:
  :+
  :magic
  :science

julia> r2.funcdefs_without_signatures
Set{Symbol} with 1 element:
  :weather

Low-level: SymbolsState

If you are not interested in just the dependencies between expressions, there is a more low-level data structure available. (We include it for completeness, but Pluto does not use this data, except to generate a ReactiveNode.)

The function compute_symbols_state take an expression as argument, and returns a SymbolsState.

Base.@kwdef mutable struct SymbolsState
    references::Set{Symbol} = Set{Symbol}()
    assignments::Set{Symbol} = Set{Symbol}()
    funccalls::Set{FunctionName} = Set{FunctionName}()
    funcdefs::Dict{FunctionNameSignaturePair,SymbolsState} = Dict{FunctionNameSignaturePair,SymbolsState}()
    macrocalls::Set{FunctionName} = Set{FunctionName}()
end

with

const FunctionName = Vector{Symbol}

struct FunctionNameSignaturePair
    name::FunctionName
    signature_hash::UInt
end

FunctionNameSignaturePair looks like FunctionNameSignaturePair([:Base, :sqrt], UInt(0xb187232b478)). It contains a "hash of the function signature, minus variable names", i.e. Base.sqrt(x::Int)::String and Base.sqrt(x::Number) will have different hashes, but Base.sqrt(x) and Base.sqrt(woww) won't.

Example for compute_symbols_state

julia> using ExpressionExplorer

julia> compute_symbols_state(:(a = b + c))
SymbolsState(
    references=Set([:b, :c]), 
    assignments=Set([:a]), 
    funccalls=Set([[:+]]), 
    funcdefs=Dict{ExpressionExplorer.FunctionNameSignaturePair, SymbolsState}(), 
    macrocalls=Set{Vector{Symbol}}()
)

julia> compute_symbols_state(:(a = b))
SymbolsState(
    references=Set([:b]), 
    assignments=Set([:a]), 
    funccalls=Set{Vector{Symbol}}(), 
    funcdefs=Dict{ExpressionExplorer.FunctionNameSignaturePair, SymbolsState}(), 
    macrocalls=Set{Vector{Symbol}}()
)

julia> compute_symbols_state(:(a(b) = b + c))
SymbolsState(
    references=Set{Symbol}(), 
    assignments=Set{Symbol}(), 
    funccalls=Set{Vector{Symbol}}(), 
    funcdefs=Dict{ExpressionExplorer.FunctionNameSignaturePair, SymbolsState}(
        ExpressionExplorer.FunctionNameSignaturePair([:a], 0x4e081629cf5e5d05) => 
            SymbolsState(
                references=Set([:c]), 
                assignments=Set{Symbol}(), 
                funccalls=Set([[:+]]), 
                funcdefs=Dict{ExpressionExplorer.FunctionNameSignaturePair, SymbolsState}(), 
                macrocalls=Set{Vector{Symbol}}()
            )
        ), 
    macrocalls=Set{Vector{Symbol}}()
)

Macro calls

ExpressionExplorer ignores the arguments of macro calls. Macros can transform an expression into anything, so the output of ExpressionExplorer for expressions with a macro call is ambiguous. For example, the expression @time x contains a reference to x, while @gensym x contains a definition of x.

In this example, notice that the assignment to x and reference to y are detected, but AAA and BBB are ignored, because they happen inside a macro call argument.

julia> ExpressionExplorer.compute_reactive_node(quote
           x = y
           @time AAA = BBB
       end)
ReactiveNode(Set([Symbol("@time"), :y]), Set([:x]), Set{Symbol}(), Set{FunctionNameSignaturePair}(), Set{Symbol}(), Set([Symbol("@time")]))

You can check whether there were any unexplored macro call arguments with the .macrocalls field of the ReactiveNode, which should be an empty set.

To solve this, you can macroexpand expressions before giving them to ExpressionExplorer. For example:

julia> ExpressionExplorer.compute_reactive_node(macroexpand(Main, quote
           x = y
           @time AAA = BBB
       end))
ReactiveNode(Set([:first, :GC_Diff, :isnothing, :gc_alloc_count, :-, :gc_num, :cumulative_compile_time_ns, :time_ns, :y, :BBB, :print, :cumulative_compile_timing, :time_print, :last, :!]), Set([:AAA, :x]), Set{Symbol}(), Set{FunctionNameSignaturePair}(), Set{Symbol}(), Set{Symbol}())

Notice that now, AAA and BBB are detected, along with functions used inside the @time expression.

Utility functions

The package also includes some utility functions used by Pluto.jl, that might also be useful to other packages.

compute_usings_imports

With compute_usings_imports you can extract all using or import expressions contained in a larger expression.

julia> ex = quote
           if something
               import A.B: c
           else
               using D
           end
       end
quote;

julia> result = compute_usings_imports(ex);

julia> result.usings
Set{Expr} with 1 element:
  :(using D)

julia> result.imports
Set{Expr} with 1 element:
  :(import A.B: c)

This function is used by Pluto's built-in package manager to learn which packages are used in a notebook.

get_rootassignee

get_rootassignee(ex)::Union{Symbol,Nothing}

If the expression is a (simple) assignment at its root, return the assignee as Symbol, return nothing otherwise.

is_toplevel_expr

is_toplevel_expr(ex)::Bool

Return whether the expression is of the form Expr(:toplevel, LineNumberNode(..), any).