Is it possible to write a constraint to exclude a certain type?

php editor Strawberry brings you an article about writing constraints to exclude certain types. In PHP, we can use type constraints to limit the parameter types of functions or methods, but is it possible to write a constraint to exclude a certain type? The answer is yes. By using the "!" operator, we can exclude certain types from type constraints, thereby achieving more precise parameter type restrictions. In this article, we'll detail how to use the "!" operator to exclude a type, and give practical code examples. Let’s explore this interesting topic together!

Question content

type nonstring interface {
    // ???
}

func myfunc[t nonstring](v t) t {
    return v
}

func main() {
    myfunc(1)          // ok
    myfunc(false)      // ok
    myfunc([]string{}) // ok

    myfunc("hi, boy!") // i hope a comiler error occurs here!  
}

What I want is described in the comments of the code above.

Is it possible in golang?

Original scene

I have two functions as follows:

func Any2String(v any) string {        
    return fmt.Sprint(v) // Slow version
}

func Integer2String[N Integer](v N) string {
    return strconv.FormatInt(int64(v), 10) // Fast version
}

any2string(v) can convert any v to a string. However, if v is an integer, integer2string(v) is much faster.

Therefore, I would like the compiler to prevent me from calling any2string(v) when the faster integer2string(v) is applicable.

Solution

No, this is not possible. Constraint type sets cannot be expressed as set subtraction.

You can define constraints that cannot be satisfied by any type, for example:

type notsatisfiable interface {
    int
    foo()
}

This constraint requires that the type be exactly int and have method foo(). Since you cannot define methods on predeclared types, it is impossible to satisfy this constraint.

However, you cannot define constraints like "any-minus-int" or "any-plus-notsatisfiable".

If your function is identical for any type except discrete quantity types, just use the type switch:

func ToString(v any) string {
    switch t := v.(type) {
    case int:
        return Integer2String(t)
    default:
        return Any2String(t)
    }
}

You don't even need generics, since regular parameters of any type will work just fine.

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