Can I create an alias for a generic function? I get error "Cannot use generic function without instantiation"

Question

I can define a generic function:

package hello

func IsZero[T int64|float64](value T) bool {
   return value == 0
}

Then if I try to alias that function in another package, it fails:

package world

import "hello"

var IsZero = hello.IsZero

The above doesn't compile with:

cannot use generic function hello.IsZero without instantiation

Instead this works:

var IsZero = hello.IsZero[int64]

Is it possible to do this, using some other syntax?

Answer 1

That's not an alias. And you already have your answer, actually. But if you want a formal reference, from the language specs, Instantiations:

A generic function that is is not called requires a type argument list for instantiation

So when you attempt to initialize a variable of function type, the function hello.IsZero is not called, and therefore requires instantiation with specific type parameters:

// not called, instantiated with int64
var IsZero = hello.IsZero[int64]

At this point the variable (let's give it a different name for clarity) zeroFunc has a concrete function type:

    var zeroFunc = IsZero[int64]
    fmt.Printf("type: %T\n", zeroFunc) 

Prints:

type: func(int64) bool

This might or might not be what you want, as this effectively monomorphises the function.

If you just want to have a local symbol, with the same implementation (or a tweaked version thereof), declaring a "wrapper" function works. Just remember that the type parameters of your wrapper can only be as strict or stricter than the wrapped one's

E.g. Given

IsZero[T int64 | float64](v T)

your wrapper can not be

WrapIsZeroPermissive[T int64 | float64 | complex128](v T) bool {
    return IsZero(v) // does not compile, T's type set is a superset
}

but can be

WrapIsZeroStricter[T int64](v T) bool {
    return IsZero(v) // ok, T's type set is a subset
}