Stream: helpdesk (published)

Topic: Capturing types of variables within a macro

Brenhin Keller (Jun 03 2022 at 22:23):

Say I want to write a macro

@foo f(a,b,c)

that captures and modifies a function call f(a,b,c). Is there any way to get the types of the arguments a,b, etc. within the macro?

Brenhin Keller (Jun 03 2022 at 22:27):

Within the macro they're all just Symbols within the captured Expr, so I can't just use typeof directly...

Brenhin Keller (Jun 03 2022 at 22:28):

The only way I can think of is something along the lines of

macro bad_form(expr)
    argtypes = ntuple(i->typeof(eval(expr.args[i+1])), length(expr.args)-1)
    :($argtypes)
end

Brenhin Keller (Jun 03 2022 at 22:29):

julia> a, b, c = 1, 2.0, 3//4
(1, 2.0, 3//4)

julia> @bad_form f(a,b,c)
(Int64, Float64, Rational{Int64})

which works in global scope but is a massive code smell AFAIU

Brenhin Keller (Jun 03 2022 at 22:33):

N.b. I can't do something like

macro good_form_but_not_helpful(expr)
    :(typeof($(expr.args[2])))
end

because I need access to the types within the body of the macro

Mosè Giordano (Jun 04 2022 at 01:24):

Don't the @code_* macros do exactly this? How do they work? Do they use the generated function suggested above? I'm away from computer, can't easily check myself.

Note that @ccall shouldn't infer the type of the arguments, you can pass a value with a different type as long as it can be converted to the actual type required by the library

Mosè Giordano (Jun 04 2022 at 01:36):

Ok, I browsed a bit the repo on the phone and found it: https://github.com/JuliaLang/julia/blob/bd8dbc388c7b89f68838ca554ed7ba91740cce75/stdlib/InteractiveUtils/src/macros.jl#L34

Mosè Giordano (Jun 04 2022 at 01:39):

I don't see any generated function anywhere

Brenhin Keller (Jun 04 2022 at 01:52):

Oh woah!

Notification Bot (Jun 04 2022 at 01:52):

Brenhin Keller has marked this topic as unresolved.

Mason Protter (Jun 04 2022 at 02:32):

Mosè Giordano said:

Don't the @code_* macros do exactly this? How do they work? Do they use the generated function suggested above? I'm away from computer, can't easily check myself.

Note that @ccall shouldn't infer the type of the arguments, you can pass a value with a different type as long as it can be converted to the actual type required by the library

The @code_* macros do not do anything with the type in their body.

Last updated: Oct 02 2023 at 04:34 UTC