The following looks like a bug in (undecidable) instances resolution. {-# LANGUAGE MultiParamTypeClasses,FlexibleInstances,UndecidableInstances, OverlappingInstances,IncoherentInstances #-} class C a b instance C a (a,b) class D a b instance (D a b, C b c) => D a c data Foo = Foo deriving Show data Bar = Bar deriving Show instance D Foo Foo c :: C x y => x -> y -> () c _ _ = () d :: D x y => x -> y -> () d _ _ = () *Main> d Foo Foo () -- as expected *Main> c Foo (Foo,Bar) () -- as expected *Main> d Foo (Foo,Bar) <interactive>:1:1: Context reduction stack overflow; size = 21 Use -fcontext-stack=N to increase stack size to N [skip] $dD :: D Foo b1 $dD :: D Foo b $dD :: D Foo (Foo, Bar) In the expression: d Foo (Foo, Bar) In an equation for `it': it = d Foo (Foo, Bar) I.e. for some reason on the second step resolver fails to pick up the most specific (and the most obvious) instance D Foo Foo and continues to apply the instance (D a b, C b c) => D a c. Reproduced with GHC 6.12.1 and 7.0.1. -- Roman I. Cheplyaka :: http://ro-che.info/ Don't worry what people think, they don't do it very often.