CODEWITHSUNDEEP
juliaaliasfunction-composition
Alec
Alec

Reputation: 4482

How to alias composite function with keyword arguments?

I can alias a single function like so:

julia> f(y;x=1) = x * y
f (generic function with 2 methods)
julia> const g = f
f (generic function with 1 method)
julia> g(3,x=2)
6

But if I try to do that with a composite function, the kwarg causes trouble:

julia> const gg= sqrt ∘ f
#62 (generic function with 1 method)
julia> gg(3,x=2)
ERROR: MethodError: no method matching (::Base.var"#62#63"{typeof(sqrt),typeof(f)})(::Int64; x=2)
Closest candidates are:
  #62(::Any...) at operators.jl:875 got unsupported keyword argument "x"

Is there a solution to this? I am trying to pass the arguments to f and then transform that result via a shortcut (gg in the above MWE).

Upvotes: 2

Views: 206

Answers (1)

Bogumił Kamiński
Bogumił Kamiński

Reputation: 69949

You have the following definition:

∘(f, g) = (x...)->f(g(x...))

and as you can see it does not support keyword arguments. Probably you can open an issue to discuss allowing them.

For now you can define your own version of it like this:

julia> ⋄(f, g) = (x...; kw...)->f(g(x...; kw...))
⋄ (generic function with 1 method)

julia> f(y;x=1) = x * y
f (generic function with 1 method)

julia> const gg = sqrt ⋄ f
#1 (generic function with 1 method)

julia> gg(3,x=2)
2.449489742783178

EDIT:

I have switched to the other OS, and I see that ⋄ does not render very sharp here. I used the following symbol:

help?> ⋄
"⋄" can be typed by \diamond<tab>

which is defined to be allowed to be an infix operator and is relatively easy to remember while not overshadowing ∘.

Upvotes: 7

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