Hi cafè, given the following toy code: --------------- module Main where class Dumb p where dumb :: p -> String newtype Branded a b = Branded b unbrand :: Branded a b -> b unbrand (Branded x) = x wrong :: Dumb a => b -> Branded a b wrong = Branded right :: Show a => b -> Branded a b right = Branded --------------- Why: --------------- quarry:Haskell paris$ ghci -O1 GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help *Main> unbrand $ right True True *Main> unbrand $ right "Foo" "Foo" --------------- but: --------------- *Main> unbrand $ wrong True <interactive>:1:10: Ambiguous type variable `a' in the constraint: `Dumb a' arising from a use of `wrong' at <interactive>:1:10-19 Probable fix: add a type signature that fixes these type variable(s) --------------- ? Maybe it's a dumb question but... thank you for any explanation... -- Cristiano GPG Key: 4096R/C17E53C6 2010-02-22 Fingerprint = 4575 4FB5 DC8E 7641 D3D8 8EBE DF59 B4E9 C17E 53C6