Matthias Fischmann wrote:
instance (Monad m, HSequence m HNil HNil) => HSequence m HNil HNil where hSequence _ = return HNil
how can i use the goal of the declaration as one of the conditions without causing some sort of black hole in the type inference algorithm?
Very easily: the instance head is implicitly the part of its own context (so that a method can be recursive). A simple way to see that is the following deliberately erroneous class:
class C a where mc :: a -> Bool instance Eq a => C a where mc x = x > x
The error message says Could not deduce (Ord a) from the context (C a, Eq a) arising from use of `>' at /tmp/f2.hs:30:36 It is revealing to observe the context that the typechecker thinks is available: it is (C a, Eq a). "Eq a" is there because we explicitly wrote it in the instance declaration. C a is there just by default. We could just as well written
instance (Ord a, C a) => C a where mc x = x > x
Incidentally, the hSequence can be written as follows
import TypeCastGeneric2 data ConsM
instance (TypeCast (m1 l) (m l), Monad m) => Apply ConsM (m a, m1 l) (m (HCons a l)) where apply _ (me,ml) = liftM2 HCons me (typeCast ml)
hSequence l = hFoldr (undefined::ConsM) (return HNil) l
hlist = HCons (Just 1) (HCons (Just 'c') HNil) hlist2 = HCons ([1]) (HCons (['c']) HNil) testHSequence = hSequence hlist testHSequence2 = hSequence hlist2
*Foo> :t testHSequence testHSequence :: Maybe (HCons Integer (HCons Char HNil)) *Foo> testHSequence Just (HCons 1 (HCons 'c' HNil)) *Foo> testHSequence2 [HCons 1 (HCons 'c' HNil)] The typechecker will complain if we try to mix different monads within the same HList, and then sequence it.