Hi,

What's the best way, when using a Callable as a template parameter, to constrain it by signature?

For example, if you have a function that sorts an array like so:

T[] sortArray(alias less, T)(T[] arr) { ... }

Then you know that you'd want `less` to have signature (T, T) -> bool.

Now, I can use something like isCallable std.traits to make sure the predicate is a Callable, and there are various function traits in the module that I could combine with `is` clauses to enforce the signature it seems, but that seems very clunky.  Is there a better way?

I note that I don't find many examples of this in phobos, so I'm wondering if there actually is a good solution...

Thanks,
-c.

On 6/7/16 9:42 PM, Carl Vogel wrote:
> Hi,
>
> What's the best way, when using a Callable as a template parameter, to
> constrain it by signature?
>
> For example, if you have a function that sorts an array like so:
>
> T[] sortArray(alias less, T)(T[] arr) { ... }
>
> Then you know that you'd want `less` to have signature (T, T) -> bool.
>
> Now, I can use something like isCallable std.traits to make sure the
> predicate is a Callable, and there are various function traits in the
> module that I could combine with `is` clauses to enforce the signature
> it seems, but that seems very clunky.  Is there a better way?
>
> I note that I don't find many examples of this in phobos, so I'm
> wondering if there actually is a good solution...

The de-facto way to do this is to write a lambda, and check that you can call the function inside the lambda, then check to see if that compiles.

e.g.:

if(is(typeof( { bool b = less(T.init, T.init)); } )))

The is(typeof( construct means "does this have a valid type". Should only work if the call is sound.

You can also use __traits(compiles, but for some reason Phobos writers prefer the is(typeof( mechanism (I remember someone saying there is a difference, but I don't know what it is).

-Steve

The other way is better, but since you asked...

On Wednesday, 8 June 2016 at 01:42:55 UTC, Carl Vogel wrote:
> Now, I can use something like isCallable std.traits to make sure the predicate is a Callable, and there are various function traits in the module that I could combine with `is` clauses to enforce the signature it seems, but that seems very clunky.  Is there a better way?

You could put all that stuff into one single template like this:

template isCompatibleCallable(alias Src, alias Dest) {
     static assert(isSomeFunction!Src || isCallable!Src,
                                   "Source is not callable");
     static assert(isSomeFunction!Dest || isCallable!Dest,
                                   "Destination is not callable");

  static assert(is(ParameterTypeTuple!Src == ParameterTypeTuple!Dest),
                                "Type Tuples differ");
  pragma(msg,ParameterStorageClassTuple!Src ==
                                ParameterStorageClassTuple!Dest);
  static assert(ParameterStorageClassTuple!Src ==
                                ParameterStorageClassTuple!Dest,
                                "Storage classes differ");
  static assert(is(ReturnType!Src == ReturnType!Dest),
                                "Return type differs");
  immutable bool isCompatibleFunction = true;
}

That works if you have a "default action" when the callable isn't specified, because you can match the Callable to the default one. So like...

bool default_less(T)(T a, T b) { return a < b; }

T[] sortArray(alias less = default_less!T, T)(T[] arr) if(isCompatibleCallable(less,default_less!T) { ... }

But it's probably clearer to use that is(typeof({ how this function will be called })) trick.

On Thursday, 9 June 2016 at 19:08:52 UTC, cy wrote:
> But it's probably clearer to use that is(typeof({ how this function will be called })) trick.

A very delayed thanks to both of you. It does seem like it would be useful to have something like a hasSignature!(Fun, Ret, Args...) defined in std.traits. But Steven's solution is pretty good.

Thanks again!