Hi,
here is how you do sequencing for HList, and a question why the type
signatures are valid. Here is the code:
{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-undecidable-instances #-}
module Foo where
import Char
import HListPrelude
class (Monad m, HList l) => HSequence m l l' | l -> m l'
where hSequence :: l -> m l'
instance (Monad m, HSequence m HNil HNil) => HSequence m HNil HNil
where hSequence _ = return HNil
instance (Monad m, HSequence m l l') => HSequence m (HCons (m a) l) (HCons a l')
where hSequence (HCons ma ml) = do
a <- ma
l <- hSequence ml
return (HCons a l)
hlist = HCons (Just 1) (HCons (Just 'c') HNil)
testHSequence = hSequence hlist
*Foo> testHSequence
Just (HCons 1 (HCons 'c' HNil)) :: Maybe (HCons Integer (HCons Char HNil))
what staggers me is the instance declaration of "HSequence m HNil
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?
also i wanted to show off with the code :-). should i submit it
somewhere?
cheers,
matthias
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