Re: [RFC] Bang constraint

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 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 [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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