Function output parametric type assertion

General Usage
1

So, I can assert the output type of a function like this (trivial example for demo purposes)

 f(x)::Int = x

However, what if the output type depends on an input type parameter

f(x::Vector{T})::T where T = x[1]

This produces an error and I don’t understand how I can assert the output type T correctly here.

How can I assert that the output type of a function is the input parameter T in general?

2

I think this is a problem with the short function definition (https://github.com/JuliaLang/julia/issues/21847 and https://github.com/JuliaLang/julia/issues/31560)

The long version works as expected:

julia> function f(x::T)::T where {T}
         return x*1.0::T
       end
f (generic function with 1 method)

julia> f(2)
ERROR: TypeError: in typeassert, expected Int64, got Float64
Stacktrace:
 [1] f(::Int64) at ./REPL[4]:2
 [2] top-level scope at none:0
1 Like
3

This seems to work:

julia> (f(x::Vector{T})::T) where {T} = x[1]
f (generic function with 1 method)

julia> f([3, 4])
3
2 Likes