Note though, it doesn't mean the same thing to say (Foo a, Bar a b) => ... as it does to say
Foo a => Bar a b => ... The latter can use Foo a when working on Bar a b, but not Bar a b to discharge Foo a, which makes a difference when you have functional dependencies. So in some sense the 'pattern requires/supplies' split is just that. That said, Richard's other option pattern Foo a => P :: Bar a => a has the benefit that it looks a bit like the old datatype contexts (but here applied to the constructor/pattern). If we expect the left hand side or the right hand side to be most often trivial then that may be worth considering. You'd occasionally have things like pattern (Num a, Eq a) => Foo :: a for pattern Foo = 8 but most of the time they'd wind up just looking like a GADT constructor. -Edward On Sun, Nov 9, 2014 at 10:02 PM, Richard Eisenberg <[email protected]> wrote: > > On Nov 9, 2014, at 2:11 PM, Simon Peyton Jones <[email protected]> > wrote: > > > > * One other possibility would be two => thus > > pattern P :: (Eq b) => (Num a, Eq a) => ...blha... > > > > I should note that I can say this in 7.8.3: > > foo :: Show a => Eq a => a -> String > foo x = show x ++ show (x == x) > > Note that I've separated the two constraints with a =>, not a comma. This > syntax does what you might expect. (I actually believe that this is an > improvement over the conventional syntax, but that's a story for another > day.) For better or worse, this trick does not work for GADT constructors > (which is a weird incongruence with function type signatures), so adding > the extra arrow does not really steal syntax from GADT pattern synonyms. > > Richard > _______________________________________________ > ghc-devs mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/ghc-devs >
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