gistfile1.txt

[1 of 1] Compiling Test             ( Test.hs, Test.o )

Test.hs:17:10:
    Illegal instance declaration for `Heap (CacheHeap h) a'
      (the Coverage Condition fails for one of the functional dependencies;
       Use -XUndecidableInstances to permit this)
    In the instance declaration for `Heap (CacheHeap h) a'

test.hs

{-# LANGUAGE GADTs, MultiParamTypeClasses, FlexibleInstances, FunctionalDependencies #-}

module Test where

class Ord a => Heap h a | h -> a where
    empty :: h
    insert :: a -> h -> h
    findMin :: h -> Maybe a
    deleteMin :: h -> h
    toList :: h -> [a]
    fromList :: [a] -> h
    fromList = foldr insert empty

data CacheHeap h where
    KacheHeap :: (Heap h a) => h -> Maybe a -> CacheHeap h

instance (Heap h a) => Heap (CacheHeap h) a where
    empty = KacheHeap (empty :: h a) Nothing