I'll try to be straight.

I have the following function:

>public size_t indexOf(alias pred = "a == b", Range)(Range array)
>{
>	alias predicate = unaryFun!pred;
>	for(size_t i = 0; i < array.length; i++)
>		if(predicate(array[i]))
>			return i;
>	return size_t.max;
>}

Say that I may want to use the function in a @nogc scope, providing a @nogc callable, but I also want to reuse the same code in managed scope while passing a delegate: how can I apply the attributes of the given callable to the function so that they're automatically determined at compile time?

I know of std.traits.SetFunctionAttributes and std.traits.FunctionAttributes, but I have no idea on how to apply them to solve this. Perhaps I should declare indexOf as a template?

On Wed, Jun 12, 2019 at 07:46:12PM +0000, Mek101 via Digitalmars-d-learn wrote: [...]
> > public size_t indexOf(alias pred = "a == b", Range)(Range array)
> > {
> > 	alias predicate = unaryFun!pred;
> > 	for(size_t i = 0; i < array.length; i++)
> > 		if(predicate(array[i]))
> > 			return i;
> > 	return size_t.max;
> > }
> 
> Say that I may want to use the function in a @nogc scope, providing a @nogc callable, but I also want to reuse the same code in managed scope while passing a delegate: how can I apply the attributes of the given callable to the function so that they're automatically determined at compile time?
[...]

You don't need to do anything special; indexOf, as you declared it, is already a template function, so the compiler should automatically apply attribute inference to it. So if pred is @nogc, and the implementation of indexOf itself doesn't invoke the GC, the compiler should automatically infer @nogc for you.

One safeguard that you might want to consider is to write a @nogc unittest, something like this:

	@nogc unittest
	{
		...
		auto result = indexOf!(...)(...);
		...
	}

The @nogc annotation on the unittest ensures that as long as the pred argument to indexOf is @nogc, indexOf itself will also be @nogc. This prevents future changes from accidentally introducing GC dependent code in the implementation of indexOf, while at the same time not explicitly marking indexOf as @nogc (only the unittest is annotated, not the function itself) allows you to use it with GC-dependent predicates as well.


T

-- 
Which is worse: ignorance or apathy? Who knows? Who cares? -- Erich Schubert

I didn't know it applied to templates other than lambdas.

Thank you for your explanation.

On 2019-06-12 22:42, Mek101 wrote:
> I didn't know it applied to templates other than lambdas.
> 
> Thank you for your explanation.

It applies to templates, lambdas (which basically are templates) and nested functions.

-- 
/Jacob Carlborg

On Thursday, June 13, 2019 3:49:04 AM MDT Jacob Carlborg via Digitalmars-d- learn wrote:
> On 2019-06-12 22:42, Mek101 wrote:
> > I didn't know it applied to templates other than lambdas.
> >
> > Thank you for your explanation.
>
> It applies to templates, lambdas (which basically are templates) and
> nested functions.

It also now applies to auto return functions, though that's a more recent change.

- Jonathan M Davis