[Haskell-cafe] Problems instancing a class
roberto.zunino at sns.it
Fri Feb 17 13:44:04 EST 2006
Juan Carlos Arevalo Baeza wrote:
> type Context = HT.HashTable String String
> 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 .
> 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.
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