[Haskell-cafe] Having trouble with instance context

Antoine Latter aslatter at gmail.com
Wed Feb 23 05:53:49 CET 2011

On Tue, Feb 22, 2011 at 8:37 PM, Kurt Stutsman <kstutsman at gmail.com> wrote:
> 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'

As the error says, compiling with the flag '-XFlexibleInstances' will
make the message go away.

You can also add a language pragma to the top of your source file:

{-# LANGUAGE FlexibleInstances #-}


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