So, what exactly does this constraint mean outside of data declarations?
E.g. how do we interpret
f :: (!a) => a -> a -> a
?
* Gabor Greif
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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