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Updated Last
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Started In
January 2020


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ExprTools provides tooling for working with Julia expressions during metaprogramming. This package aims to provide light-weight performant tooling without requiring additional package dependencies.

Alternatively see the MacroTools package for a more powerful set of tools.

Currently, this package provides the splitdef, signature and combinedef functions which are useful for inspecting and manipulating function definition expressions.

  • splitdef works on a function definition expression and returns a Dict of its parts.
  • combinedef takes a Dict from splitdef and builds it into an expression.
  • signature works on a Method, or the type-tuple sig field of a method, returning a similar Dict that holds the parts of the expressions that would form its signature.

As well as several helpers that are useful in combination with them.

  • args_tuple_expr applies to a Dict from splitdef or signature to generate an expression for a tuple of its arguments.
  • parameters which return the type-parameters of a type, and so is useful for working with the type-tuple that comes out of the sig field of a Method


julia> using ExprTools

julia> ex = :(
           function Base.f(x::T, y::T) where T
               x + y
:(function Base.f(x::T, y::T) where T
      #= none:3 =#
      x + y

julia> def = splitdef(ex)
Dict{Symbol,Any} with 5 entries:
  :args        => Any[:(x::T), :(y::T)]
  :body        => quote:name        => :(Base.f)
  :head        => :function
  :whereparams => Any[:T]

julia> def[:name] = :g;

julia> def[:head] = :(=);

julia> args_tuple_expr(def)
:((x, y))

julia> def[:body] = :(*($(args_tuple_expr(def))...));

julia> g_expr = combinedef(def)
:((g(x::T, y::T) where T) = (*)((x, y)...))

julia> eval(g_expr)
g (generic function with 1 method)

julia> g_method = first(methods(g))
g(x::T, y::T) where T in Main

julia> parameters(g_method.sig)
svec(typeof(g), T, T)

julia> signature(g_method)
Dict{Symbol, Any} with 3 entries:
  :name        => :g
  :args        => Expr[:(x::T), :(y::T)]
  :whereparams => Any[:T]

JuliaCon 2021 Video

"ExprTools: Metaprogramming from reflection" by Lyndon White

YouTube Video