Hi, Seems that Haskell allows to specify "dummy" type variables in a declaration of a type synonym, which do not appear in its right-hand side. This can lead to interesting effects, which appears differently in GHC and Hugs. I would like to know, what behavior is correct according to the haskell 98 report. 1) ------------------------------------------ type F a = Int class A a where foo :: A b => a (F b) ------------------------------------------ GHC - OK Hugs - Illegal type "F b" in constructor application 2) ------------------------------------------ type F a = Int class A a where foo :: F a instance A Bool where foo = 1 instance A Char where foo = 2 xs = [foo :: F Bool, foo :: F Char] ------------------------------------------ GHC: M.hs:14:6: Ambiguous type variable `a' in the constraint: `A a' arising from a use of `foo' at M.hs:14:6-8 Probable fix: add a type signature that fixes these type variable(s) M.hs:14:21: Ambiguous type variable `a1' in the constraint: `A a1' arising from a use of `foo' at M.hs:14:21-23 Probable fix: add a type signature that fixes these type variable(s) Hugs: [1,2] Thanks, Vladimir