Can't return generated generic/type-taking functions

[tgschultz]

const IsTypeFn = fn(type) bool;

fn generateIsType(comptime T: type) IsTypeFn
{
    const Closure = struct
    {
        pub fn is(comptime Other: type) bool
        {
            return T == Other;
        }
    };
    return Closure.is;
}

const isU32 = generateIsType(u32);

test "generic fn generation"
{
    debug.assert(isU32(u32));
}

error: expected type 'fn(type) bool', found 'fn(type)var'

This appears to be because of the kind of type-erasure zig performs as a result of being able to do things like this fn func(comptime T: type, x: T) T, where any type after the first generic (or type) argument could be inferred. Since var does not work as a return type (#447), the only way around this is to return the Closure struct, itself a workaround for the inability to nest functions directly within other functions (#229), and take the Fn at the callsite:

const IsTypeFn = fn(type) bool;

fn generateIsType(comptime T: type) type
{
    const Closure = struct
    {
        pub fn is(comptime Other: type) bool
        {
            return T == Other;
        }
    };
    return Closure;
}

const isU32 = generateIsType(u32).is;

test "generic fn generation"
{
    debug.assert(isU32(u32));
}

Test 1/1 generic fn generation...OK
All tests passed.

[tgschultz]

It turns out that it is possible, you just have to trick the compiler a little bit:

fn typeFnWorkaround(comptime T: type) bool { return false; }
const IsTypeFn = @typeOf(typeFnWorkaround);

fn generateIsType(comptime T: type) IsTypeFn
{
    const Closure = struct
    {
        pub fn is(comptime Other: type) bool
        {
            return T == Other;
        }
    };
    return Closure.is;
}

const isU32 = generateIsType(u32);

test "generic fn generation"
{
    debug.assert(isU32(u32));
}

Test 1/1 generic fn generation...OK
All tests passed.

[Vexu]

All these examples work currently.