Nick Name wrote:

Hi all, I have an example wich I don't understand:

First of all, let's rename the constructors and types a bit to make things clearer add the instance in question, and remove the type signatures: ---------------------------------------------------------------- module Main where class C t data T = MkT instance C T instance C () data C t => T1 t = MkT1 f1 = MkT1 data C t => T2 t = MkT2 t f2 = MkT2 () ---------------------------------------------------------------- Then we can easily ask GHC: ---------------------------------------------------------------- panne@jeanluc:~> ghci -v0 Main.hs *Main> :i T1 MkT1 f1 T2 MkT2 f2 -- T1 is a type constructor, defined at Main.hs:8 data (C t) => T1 t = MkT1 -- MkT1 is a data constructor, defined at Main.hs:8 MkT1 :: forall t. T1 t -- f1 is a variable, defined at Main.hs:10 f1 :: forall t. T1 t -- T2 is a type constructor, defined at Main.hs:12 data (C t) => T2 t = MkT2 t -- MkT2 is a data constructor, defined at Main.hs:12 MkT2 :: forall t. (C t) => t -> T2 t -- f2 is a variable, defined at Main.hs:14 f2 :: T2 () ----------------------------------------------------------------

The first function, f1, is accepted both by hugs and ghc, unlike the second wich is rejected.

Why does this happen? Shouldn't f1 be rejected with "no instance C ()"

The reason is buried in http://haskell.org/onlinereport/decls.html#sect4.2.1 In a nutshell: The context in datatype declarations has only an effect for the *data* constructors of that type which use the type variables mentioned in the context. Contexts have no effect for the *type* constructor. IIRC the reason for this design decision was that contexts in type signatures should always be explicit. Cheers, S.