Re: [Haskell-cafe] type/class question: toString

6 Nov 2007

      On 11/6/07, Graham Fawcett  wrote:
...
ToString.hs:5:0:
    Illegal instance declaration for `MyShow String'
        (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 `MyShow String'
ToString.hs:6:0:
    Illegal instance declaration for `MyShow a'
        (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 `MyShow a'
In ghc 6.8.1, the error messages are more helpful:

foo.hs:5:0:
    Illegal instance declaration for `MyShow String'
        (All instance types must be of the form (T t1 ... tn)
         where T is not a synonym.
         Use -XTypeSynonymInstances if you want to disable this.)
    In the instance declaration for `MyShow String'

foo.hs:6:0:
    Illegal instance declaration for `MyShow a'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are distinct type *variables*
         Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `MyShow a'

When I run with -XTypeSynonymInstances -XFlexibleInstances it works.

Re: [Haskell-cafe] type/class question: toString

David Benbennick