On Sun, Sep 25, 2016 at 7:25 AM, Marius Millea <mariusmil...@gmail.com> wrote:
> I can store a macro to a variable (let use the identity macro "id" as an
> example),
>
> julia> idmacro = macro id(ex)
> :($(esc(ex)))
> end
> @id (macro with 1 method)
>
>
> How can I use this macro now? I can *almost* do it by hand by passing an
> expression as an argument and eval'ing the result, but it doesn't work
> because there's esc's left,
Assuming this is just for understanding how macros are implemented,
you can just construct a microcall expression
julia> idmacro = macro id(ex)
esc(ex)
end
@id (macro with 1 method)
julia> idmacro(:(1 + 2))
:($(Expr(:escape, :(1 + 2))))
julia> eval(Expr(:macrocall, :idmacro, :(1 + 2)))
3
This is unlikely what you want to do in real code.
Side notes,
As shown above, you can just do `esc(ex)`, `:($foo)` is equivalent to `foo`
You can also use a variable name starts with `@` since that's the
syntax that triggers the parsing to a macrocall expression.
julia> eval(:($(Symbol("@idmacro")) = idmacro))
@id (macro with 1 method)
julia> @idmacro 1 + 2
3
julia> Meta.show_sexpr(:(@idmacro 1 + 2))
(:macrocall, Symbol("@idmacro"), (:call, :+, 1, 2))
You can do this transformation with a macro too (i.e. make @macrocall
idmacro 1 + 2 construct a macrocall expression) but it's not really
useful since you can't do this for a local variable, which is also why
I said don't do this in real code.
>
> julia> @id 1+2
> 3
>
>
> julia> idmacro(:(1+2))
> :($(Expr(:escape, :(1 + 2))))
>
>
> julia> eval(idmacro(:(1+2)))
> ERROR: syntax: unhandled expr (escape (call + 1 2))
> in eval(::Module, ::Any) at ./boot.jl:234
> in eval(::Any) at ./boot.jl:233
>
>
>
> Does Julia provide a way to use such macros stored in variables? Thanks!
