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December 06, 2012
More templated type argument inference
Currently this code is not supported:

void foo(T)(T x, T function(T) f) {}
void main() {
    foo(1, (int a) => a * a); // OK
    foo(1, a => a * a);       // Error
}


With the latest alpha compiler it gives:

test.d(4): Error: template test.foo does not match any function 
template declaration. Candidates are:
test.d(1):        test.foo(T)(T x, T function(T) f)
test.d(4): Error: template test.foo(T)(T x, T function(T) f) 
cannot deduce template function from argument types !()(int,void)


Do you think it's a good idea to allow it?

Bye,
bearophile
December 06, 2012
Re: More templated type argument inference
On Thursday, 6 December 2012 at 13:04:05 UTC, bearophile wrote:
> Currently this code is not supported:
>
> void foo(T)(T x, T function(T) f) {}
> void main() {
>     foo(1, (int a) => a * a); // OK
>     foo(1, a => a * a);       // Error
> }
>
>
> With the latest alpha compiler it gives:
>
> test.d(4): Error: template test.foo does not match any function 
> template declaration. Candidates are:
> test.d(1):        test.foo(T)(T x, T function(T) f)
> test.d(4): Error: template test.foo(T)(T x, T function(T) f) 
> cannot deduce template function from argument types 
> !()(int,void)
>
>
> Do you think it's a good idea to allow it?
>
> Bye,
> bearophile

The second is template lambda and templates have void type. What 
you suggest here?
December 06, 2012
Re: More templated type argument inference
Maxim Fomin:

> The second is template lambda and templates have void type. 
> What you suggest here?

In D template functions become functions if they are passed where 
the D compiler can infer their full type. This is correct code:


void foo(int function(int) f) {}
void main() {
    foo(a => a * a);
}


Bye,
bearophile
December 06, 2012
Re: More templated type argument inference
On Thursday, 6 December 2012 at 13:04:05 UTC, bearophile wrote:
> Currently this code is not supported:
>
> void foo(T)(T x, T function(T) f) {}
> void main() {
>     foo(1, (int a) => a * a); // OK
>     foo(1, a => a * a);       // Error
> }
>

I forgot there is int here, so T in lambda should be deduced as 
int as well.

>
> With the latest alpha compiler it gives:
>
> test.d(4): Error: template test.foo does not match any function 
> template declaration. Candidates are:
> test.d(1):        test.foo(T)(T x, T function(T) f)
> test.d(4): Error: template test.foo(T)(T x, T function(T) f) 
> cannot deduce template function from argument types 
> !()(int,void)
>
>
> Do you think it's a good idea to allow it?
>
> Bye,
> bearophile

Does makes sense.
December 06, 2012
Re: More templated type argument inference
Doesn't that mean you're asking the compiler to have a full unification
engine? Your example is simple, but the general case is more complicated:

void foo(T,U,V)(Tuple!(T,U) tup, V function(U) fun)
{ ... }

void main()
{
   foo(tuple(1,"abc"), (a) => 1.34); // so, I can deduce V to be double

   foo(tuple(1,"abc"), (int i) => i+); // error, U unified to string and
int!
}
December 06, 2012
Re: More templated type argument inference
Philippe Sigaud:

> Doesn't that mean you're asking the compiler to have a full 
> unification
> engine? Your example is simple, but the general case is more 
> complicated:

I think the general case doesn't require a whole program type 
inference, just a local one. But maybe it's too much complex to 
do it in D :-) I don't know.

Bye,
bearophile
December 07, 2012
Re: More templated type argument inference
On 12/06/2012 02:04 PM, bearophile wrote:
> Currently this code is not supported:
>
> void foo(T)(T x, T function(T) f) {}
> void main() {
>      foo(1, (int a) => a * a); // OK
>      foo(1, a => a * a);       // Error
> }
>
>
> With the latest alpha compiler it gives:
>
> test.d(4): Error: template test.foo does not match any function template
> declaration. Candidates are:
> test.d(1):        test.foo(T)(T x, T function(T) f)
> test.d(4): Error: template test.foo(T)(T x, T function(T) f) cannot
> deduce template function from argument types !()(int,void)
>
>
> Do you think it's a good idea to allow it?
>
> Bye,
> bearophile

I consider it necessary.
December 07, 2012
Re: More templated type argument inference
Timon Gehr:

> I consider it necessary.

To consider opening an enhancement request, are you able to 
explain us why? And is it possible to implement it? :-)

Bye,
bearophile
December 07, 2012
Re: More templated type argument inference
On 12/07/2012 11:12 PM, bearophile wrote:
> Timon Gehr:
>
>> I consider it necessary.
>
> To consider opening an enhancement request, are you able to explain us
> why?

It is just embarrassing if it does not work. Similar things work even in 
C#. Of course, it is harder in D because the type system is Turing 
complete and stuff.

I think we want to support things like:

[1,2,3].map(a=>2*a);

> And is it possible to implement it? :-)
>

Yes; in fact it almost works satisfactorily in my home-grown front end 
already.
As most of D compilation, it is undecidable, but the following heuristic 
should be fine:

1. Resolve and fix explicit template arguments.
2. Infer types from arguments. Unify. Do not consider known lambda
   return types at this point.
3. Analyze lambdas whose parameter types are known. Fix those types.
4. Resolve lambda return values where known. Unify.
5. Repeat 2-4 until fixed point is reached.
6. Use default template arguments.
7. Repeat 5-6 until fixed point is reached.
8. Check if everything became known and if the argument types are
   actually compatible with the inferred signature.

(Where 'fix' means, if the type should later be unified with something
that is not the same type, fail instantiation.)

Furthermore, probably it should treat curried lambdas a little more
cleverly than the above. It may be argued that this is too complex. :o)
December 07, 2012
Re: More templated type argument inference
Timon Gehr:

> I think we want to support things like:
>
> [1,2,3].map(a=>2*a);

This was one of my original use cases.


> It may be argued that this is too complex. :o)

Even if they are trying to speed up Scala compilation, it's much 
slower than Java/D compilation, because the Scala type system is 
very complex and refined and the compiler has to work a lot.
If the D type system gets refined as we are discussing here, 
maybe the compilation speed of templates will be reduced a 
little. I have no idea if this is true, and how much.

Bye,
bearophile
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