Proposal: Extension Methods on Method Groups about csharplang HOT 13 CLOSED

If I understand correctly, the proposal is to add the bold text below to the spec section 7.6.6.2:

An extension method Ci.Mj is eligible if:
• Ci is a non-generic, non-nested class
• The name of Mj is identifier
• Mj is accessible and applicable when applied to the arguments as a static method as shown above
• An implicit identity, reference, method group, anonymous function, or boxing conversion exists from expr to the type of the first parameter of Mj.

from csharplang.

@alrz I see what you mean. Because the conversion target would need to be manifest for method selection to even occur.
I think there might be other ways that this could be made to work. Basically compiler black magic that considers the possible target types having the bespoke extension method, but it would be better if there were in fact a default delegate type, or default families of delegate types.

In the black magic case (which all is in my head and I may well be insane), we are essentially saying:
take the lambda expression and the invocation of WithErrorHandling and perform a syntactic transformation on the expression such that (o => ...).WithErrorHandling is viewed, for this purpose, as DelegateExtensions.WithErrorHandling(o => ...) and bind that.
Or, another way of looking at it, are their any types to which (o => ...) is convertible having an extension method WithErrorHandling taking a single argument of that type as their first parameter.

from csharplang.

I think that's a better idea because the compiler cannot know about all possible delegate types (unless dotnet/roslyn#3990 looks up for other delegate types in scope if Action and Func were not applicable which is unlikely).

from csharplang.

(o => ...).WithErrorHandling()

Related: dotnet/roslyn#3990

from csharplang.

@sharwell Oops; fixed

from csharplang.

@alrz yes dotnet/roslyn#3990 would benefit from this, but this proposal stands on its own.

from csharplang.

@aluanhaddad Yes, but that particular example that I mentioned requires the lambda to have a default type.

from csharplang.

@SLaks Thanks, edited.

from csharplang.

Yes; that's exactly what I mean.

It would make scanning for extension methods a bit more costly (since it has to consider more conversions); I don't know how bad that is.

from csharplang.

This would be beautiful, and would make the language more functional. The WithErrorHandling() would be very appealing. The currying too.

Is it being considered for v8?

from csharplang.

@giggio It is not.

from csharplang.

Here is a use case for this, driven by #424

public static TResult Splat<T1, T2, T3, TResult>(this Func<T1, T2, T3, TResult> method, (T1, T2, T3) args)
{
    return method(args.Item1, args.Item2, args.Item3);
}

void example()
{
    var input = (1d, 1d, 1d);
    (double, double) output;

    // These both work:
    output = Extensions.Splat(Globe.ConvertToSpherical, input);
    output = ((Func<double, double, double, (double, double)>)Globe.ConvertToSpherical).Splat(input);

    // This doesn't compile. Error:
    // CS0119	'Globe.ConvertToSpherical(double, double, double)' is a method, which is not valid in the given context
    output = Globe.ConvertToSpherical.Splat(input);
}

(double theta, double psi) ConvertToSpherical(double x, double y, double z)
{
    throw new NotImplementedException();
}

from csharplang.

It would also be nice if we could do the following (automatically find the best matching type (action) for method groups):

value switch 
{
    value => value.SomeMethod,
}();

// or, if there is no native suport for this, we could at least define a method like this:

private Action AsAction(Action action)
    => action;

// and then do this:

value switch 
{
    value => value.SomeMethod.AsAction(),
}();

Because at the moment, if I have a task, I can use expression switches for "void like" meaning the void task returning nothing when awaited and have the await before the expression.

There's no reason for that limitation with void - just accepting void as a return type for an expression switch would be nice too, but I guess that's not realistic from what I've read in other issues.

Void-Task example that compiles (showing that in theory, void expressions would make just as much sense):

private Task Run(int value) => Task.CompletedTask;
private Task Run2() => Task.CompletedTask;

await (value switch 
{
    3 => Run(value),
    _ => Run2(),
});

But this does not work:

private void Run(int value) => NoOperation();
private void Run2() => NoOperation();

// Doesn't work (statement not allowed there)
value switch 
{
    3 => Run(value),
    _ => Run2(),
};

// Doesn't work (void can not be assigned to variable)
_ = value switch 
{
    3 => Run(value),
    _ => Run2(),
};

// Doesn't work (no best type found)
value switch 
{
    3 => () => Run(value),
    _ => Run2,
}();

// Does work, but is really annoying and adds noise for no reason, and you can't use expression bodies anymore if you're writing a method
Action runner = value switch 
{
    3 => () => Run(value),
    _ => Run2,
};

runner();

Allowing Extension methods on Method groups would give some flexibility, and automatically detecting that Action is the best type here (not only when running Extension methods like the proposal) would be an even bigger improvement over status quo.

Are there any updates wheter this is considered as a champion for v11 or v12?

from csharplang.