Expression interpolation

General Usage
8

I think it is probably important to note that interpolating a variables and expression in an expression lead to slightly different things.

julia> v = (1,2,3)
julia> dump(:(a in $v))
Expr
  head: Symbol call
  args: Array{Any}((3,))
    1: Symbol in
    2: Symbol a
    3: Tuple{Int64,Int64,Int64}
      1: Int64 1
      2: Int64 2
      3: Int64 3

julia> dump(:(a in (1,2,3)))

Expr
  head: Symbol call
  args: Array{Any}((3,))
    1: Symbol in
    2: Symbol a
    3: Expr
      head: Symbol tuple
      args: Array{Any}((3,))
        1: Int64 1
        2: Int64 2
        3: Int64 3

and if you don’t interpolate the expression you get.

julia> dump(:(a in :((1,2,3))))
Expr
  head: Symbol call
  args: Array{Any}((3,))
    1: Symbol in
    2: Symbol a
    3: Expr
      head: Symbol quote
      args: Array{Any}((1,))
        1: Expr
          head: Symbol tuple
          args: Array{Any}((3,))
            1: Int64 1
            2: Int64 2
            3: Int64 3

So really what it looks like interpolating the expression does is to unwrap the inner expression and pass it as an argument. I.E. The respective difference is:

julia> Expr(:call, :in, :a, (1,2,3))
:(a in (1, 2, 3))

julia> Expr(:call, :in, :a, :((1,2,3)))
:(a in (1, 2, 3))

julia> Expr(:call, :in, :a, Expr(:quote, :(1,2,3)))
:(a in $(Expr(:quote, :((1, 2, 3)))))