Re: [Haskell-cafe] Having trouble with instance context

On Tue, Feb 22, 2011 at 8:37 PM, Kurt Stutsman wrote:

Ivan Lazar Miljenovic wrote:

...

On 23 February 2011 13:10, Kurt Stutsman 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 #-} Antoine

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