Function declarations

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If the function declaration specifies type parameters, the function name denotes a generic function. A generic function must be instantiated before it can be called or used as a value.

func min[T ~int|~float64](x, y T) T {
	if x < y {
		return x
	}
	return y
}

We’ve mentioned instantiation before, but it will be covered in depth later. For now, it’s enough to show an example of what the spec means can’t be done without instantiating a generic function:

func notGeneric(a int) int { /* ... */ }

func generic[T int|float64](a T) T { /* ... */ }

var x = notGeneric // valid, x is of type func(int) int

var y = generic // invalid, until the function is instantiated, T's type is unknown, thus the function's type is undetermined

var z = generic[int] // valid, z is of type func(int) int

See this in the playground

Quotes from The Go Programming Language Specification Version of August 2, 2023