As the title says, is there a way to test the return type of the 'unaryFun' and 'binaryFun' templates from 'std.functional'?

I have the following code, and I want to to be sure that 'predicate' returns a boolean, but neither 'is(typeof(predicate) == bool)' or 'is(ReturnType!predicate == bool)' seem to work.

>private alias NullSized = Nullable!(size_t, size_t.max);

>public NullSized indexOf(alias pred = "a == b", Range, V)(Range array, V value)
>if((isRandomAccessRange!Range || isStaticArray!Range))
>{
>	alias predicate = binaryFun!pred;
>
>	for(size_t i = 0; i < array.length; i++)
>		if(predicate(array[i], value))
>			return NullSized(i);
>	return NullSized.init;
>}

On Thursday, 29 August 2019 at 08:58:18 UTC, Mek101 wrote:
> As the title says, is there a way to test the return type of the 'unaryFun' and 'binaryFun' templates from 'std.functional'?
>
> I have the following code, and I want to to be sure that 'predicate' returns a boolean, but neither 'is(typeof(predicate) == bool)' or 'is(ReturnType!predicate == bool)' seem to work.
>
>>private alias NullSized = Nullable!(size_t, size_t.max);
>
>>public NullSized indexOf(alias pred = "a == b", Range, V)(Range array, V value)
>>if((isRandomAccessRange!Range || isStaticArray!Range))
>>{
>>	alias predicate = binaryFun!pred;
>>
>>	for(size_t i = 0; i < array.length; i++)
>>		if(predicate(array[i], value))
>>			return NullSized(i);
>>	return NullSized.init;
>>}

You'll need to test calling the predicate with the actual types it will be processing - something like is(typeof(predicate(array[0], value)) == bool). You could (and quite possibly should) do this in the template constraint.

The reason is binaryFun actually returns a generic function - a template that needs to know its actual argument types. For fun, you could actually test is(typeof(predicate!(ElementType!Range, V)) == bool).

--
  Simen

I made the following enumerations to test the predicates.

>private enum bool isUnaryPredicate(alias pred, Range) = is(typeof(unaryFun!(pred)((ElementType!Range).init)) == bool);
>private enum bool isBinaryPredicate(alias pred, Range, V) = is(typeof(binaryFun!(pred)((ElementType!Range).init, V.init)) == bool);

Both work as expected. Thank you.