Stream: helpdesk (published)

Topic: Can we get the types of `args` from a function?

Davi Sales Barreira (Aug 18 2022 at 01:17):

Suppose I have a function such as:

function f(x::Int,y::Float64)::Real
      return y^x
end

Is there a way to get the domain type from this function (and possibly the "codomain", which in this case was ::Real)? I'm supposing that this function has only one method. And in case there are many, one can just take the first method asfirst(methods(f)).

I wish to get something like Tuple{Int,Float64} for the domain type, and Real for the codomain.

P.S: I already have a macro that can do this during the function declaration, but I was wondering if one can do it after the function is already declared.

Zachary P Christensen (Aug 18 2022 at 03:48):

Are you talking about looking things up in the method table?

Jakob Nybo Nissen (Aug 18 2022 at 04:56):

methods(f).sig not sure about annotated return type

Sukera (Aug 18 2022 at 05:28):

annotated return types just insert a convert at return points, it doesn't show up in the method signature (that's inference' domain)

Sukera (Aug 18 2022 at 05:28):

in general, you'll need to know the input types to your method to get the output type(s) (there may be multiple for type unstable functions)

Jakob Nybo Nissen (Aug 18 2022 at 05:30):

It also inserts a typeassert, right? But yeah okay

Sukera (Aug 18 2022 at 05:30):

yes

Mason Protter (Aug 18 2022 at 05:33):

After the convert, yeah

Davi Sales Barreira (Aug 18 2022 at 12:50):

Thanks for the replies! Here is the solution I came up with. Feel free to tell me how to improve it:

function getfunctiontype(f, methodindex=1)
    m = collect(methods(f))[methodindex]
    dom = Tuple([m.sig.types[2:end]...])
    codom = Base.code_typed(f, dom)[1].second

    if length(dom) == 1
        dom = dom[1]
    end
    return dom,codom
end

This if length(dom) == 1 is ugly, but I don't know how to avoid it.

Sundar R (Aug 18 2022 at 13:38):

Do you need it? Isn't it better to return a Tuple in all cases, do you need a special check for the one argument case?

Leandro Martínez (Aug 18 2022 at 14:41):

Just for curiosity, why are you willing to get these signatures? That feels like a XY problem, unless you are doing something very low level.

Davi Sales Barreira (Aug 18 2022 at 22:39):

Sundar R said:

Do you need it? Isn't it better to return a Tuple in all cases, do you need a special check for the one argument case?

I wrote a composition that is restricted on types, so Tuple{Int} == Int fails. Hence, I either fix this here or in the composition implementation.
I thought fixing here would be better cause it then returns the actual types.

Last updated: Oct 02 2023 at 04:34 UTC