Juan Carlos Arevalo Baeza wrote:
type Context = HT.HashTable String String [snip] Illegal instance declaration for `MyClass Context' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `MyClass Context'
"type" introduce a type synonym, and Haskell98 forbids these in instances, so GHC complains. GHC also lifts this restriction when invoked with -fglasgow-exts . http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html...
If I use "data" instead of "type", it works:
data Context = C (HT.HashTable String String)
This is fine: data (and newtype) declarations do not introduce type synonyms, but a genuine new type. Regards, Roberto Zunino.