{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances #-} {- | The HList library (C) 2004, Oleg Kiselyov, Ralf Laemmel, Keean Schupke Zipping and unzipping for (conceptually) lists of pairs. -} module Data.HList.HZip where import Data.HList.HListPrelude {-----------------------------------------------------------------------------} -- | Zip and unzip class HZip x y l | x y -> l, l -> x y where hZip :: x -> y -> l hUnzip :: l -> (x,y) {- -- Zipping version I -- | Test for zippability: equal length. class HZippable x y instance HZippable HNil HNil instance HZippable l l' => HZippable (HCons e l) (HCons e' l') -- Somehow too polymorphic. -- Version II specialises for HNil and HCons. instance HZippable x y => HZip x y (x,y) where hZip x y = (x,y) hUnzip = id -} {- -- Zipping version II -- Built-in show and alias-based type construction inconvenient. -- Version III goes for a true list of pairs. instance HZip HNil HNil (HNil,HNil) where hZip x y = (x,y) hUnzip = id instance HZip xt yt zt => HZip (HCons xh xt) (HCons yh yt) (HCons xh xt,HCons yh yt) where hZip x y = (x,y) hUnzip = id -} -- {- --- Zipping version III --- Works best for us. instance HZip HNil HNil HNil where hZip HNil HNil = HNil hUnzip HNil = (HNil,HNil) instance HZip tx ty l => HZip (HCons hx tx) (HCons hy ty) (HCons (hx,hy) l) where hZip (HCons hx tx) (HCons hy ty) = HCons (hx,hy) (hZip tx ty) hUnzip (HCons (hx,hy) l) = (HCons hx tx, HCons hy ty) where (tx,ty) = hUnzip l -- -}