[Haskell-cafe] Flexible instances

Albert Y. C. Lai trebla at vex.net
Tue Oct 14 23:51:06 EDT 2008

George Pollard wrote:
> I'm a little confused. Why is this allowed:
>> instance Eq Blah where
>>         x == y = True
> But not this:
>> Illegal instance declaration for `Stringable [Char]'
>>         (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 `Stringable [Char]'
> 'Blah' isn't a type variable, is it? Is my brain just not working right
> today?

Just in case: n=0 for "instance Eq Blah", i.e., "T a1 ... an" becomes "T".

More information about the Haskell-Cafe mailing list