[RFC] Bang constraint

[Gabor Greif]

Hi Roman,

I'd say it would be an empty constraint, since the directory
associated with (!) is empty. So this would be trivially satisfied.

There is however another possibility, something like `seq`. So both
arguments would always be normalized to WHNF before the function f is
called. Same for the result. Of course this constraint still would not
rule out

> f a b = f a b

so no big gain here.

Implementing the latter would be more ambitious, introducing basically
the same sort of 'magic' as with ! on data.

Cheers,

    Gabor

On 4/3/13, Roman Cheplyaka <roma at ro-che.info> wrote:
> So, what exactly does this constraint mean outside of data declarations?
>
> E.g. how do we interpret
>
>   f :: (!a) => a -> a -> a
>
> ?
>
> * Gabor Greif <ggreif at gmail.com> [2013-04-03 20:06:03+0200]
>> Now that Constraint kinds are pretty much into the mainstream, can we
>> consider the '!' annotation in data declarations as a magic
>> constraint? Not unlike NFData, but only ensuring that said value is in
>> WHNF.
>>
>> So the following definitions would be semantically identical
>>
>> > newtype Foo a = Foo a
>>
>> > data Foo a = Foo !a
>>
>> > data Foo a = (!a) => Foo a
>>
>> > data FooC :: (* -> Constraint) -> * -> * where
>> >   Foo :: c a => a -> FooC c a
>> > type Foo a = FooC ! a
>>
>> It just occured to me and seemed plausible at first thought, but feel
>> free to flame me because of the magic-ness.
>>
>> Cheers,
>>
>>     Gabor
>>
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>