AntC <anthony_clayden@...> writes: > > <oleg@...> writes: > > > > The headline news is that I have implemented hDeleteMany in Hugs. >
Yikes! I'd better post the code. This assumes all the usual HList infrastructure, especially class/method TypeCast as defined in-line per http://okmij.org/ftp/Haskell/typecast.html Works on Hugs version Sep 2006 -- yes! it's been hiding in plain view all these years. {- hDeleteMany does a type-indexed scan through an HList, removing all elements type `e`, even if they occur many times. Takes the standard HList idiom of 3 instances: - end of HList -- contains only HNil - HList's head contains the element of interest (HCons e l'') - HList's head not interesting, pass on (HCons e' l'') The 'interesting' instance overlaps the 'not interesting'. -} class HDeleteMany e l l' where -- no fundep hDeleteMany :: e -> l -> l' instance (TypeCast HNil l') => HDeleteMany e HNil l' where hDeleteMany e HNil = typeCast HNil -- must typeCast the result instance (HDeleteMany e l'' l') => HDeleteMany e (HCons e l'') l' where hDeleteMany e (HCons _ l'') = hDeleteMany e l'' instance (HDeleteMany e l'' l''', TypeCast (HCons e' l''') l') => HDeleteMany e (HCons e' l'') l' where hDeleteMany e (HCons e' l'') = typeCast (HCons e' (hDeleteMany e l'')) -- tests: somelist = HCons True $ HCons 'H' $ HCons "HList" $ HCons (5 :: Int) HNil somemanylist = HCons "hello" $ HCons False somelist unmanylist = hDeleteMany "bye" (hDeleteMany (undefined :: Bool) somemanylist ) -- unmanylist ===> HCons 'H' (HCons 5 HNil) AntC _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
