Hello, Consider the following code: {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, OverlappingInstances, UndecidableInstances, FunctionalDependencies #-} class Container a b | a -> b where make :: b -> a data Cont a = Cont a deriving (Show, Eq) instance Container (Cont a) a where make x = Cont x -- Part A instance (Container a b, Show a, Eq a, Num b) => Num a where fromInteger x = make (fromInteger x) d = fromInteger 3 :: Cont Integer -- Part B class Convertable a where fromInt :: Integer -> a instance Convertable Integer where fromInt = id instance (Container a b, Convertable b) => Convertable a where fromInt x = make (fromInt x) e = fromInt 3 :: Cont Integer main = do print d print e Part A and B are basically the same, the only difference being that A uses existing Num typeclass, and B creates a new one. The problem is that it compiles and works in GHC 7.2.2, but in 7.0.4 (which is the one included in the most recent Haskell Platform, that's why I bumped into this issue) part A generates the following error: t4.hs:17:31: Context reduction stack overflow; size = 21 Use -fcontext-stack=N to increase stack size to N $dNum :: Num b19 [ skipped ] $dNum :: Num b0 In the first argument of `make', namely `(fromInteger x)' In the expression: make (fromInteger x) In an equation for `fromInteger': fromInteger x = make (fromInteger x) My understanding is that in line "fromInteger x = make (fromInteger x)" compiler should take into account that 1) the signature for make is "make :: b -> a" 2) type b is known at the moment of creating instance (because a == Cont Integer, which is an instance of Container (Cont Integer) Integer). and cast "fromInteger x" to type b and pass it to make. In 7.0.4 part B (which is based on the same idea) works correctly, so it may have something to do with how fromInteger is defined in GHC guts. So, the question is, could anyone please tell me, is what I am trying to do actually correct, and what was changed from 7.0.4 to 7.2.2 to make it work? Thank you in advance. Best regards, Bogdan