[Haskell-cafe] Having trouble with instance context

Kurt Stutsman kstutsman at gmail.com
Wed Feb 23 03:37:34 CET 2011

Ivan Lazar Miljenovic wrote:
> On 23 February 2011 13:10, Kurt Stutsman <kstutsman at gmail.com> wrote:
>> I am trying to create an instance of a class for data types deriving from
>> Enum. When I try to specify this through a context on the instance
>> definition, I get an error. When I do something similar with a function, it
>> seems to work fine. Why does the instance not work?
> What's the error?
    Illegal instance declaration for `Test e'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are type *variables*,
         and each type variable appears at most once in the instance head.
         Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `Test e'

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