[Haskell-cafe] FunDeps conflict

[Brian Hulley]

Brian Hulley wrote:
> Joel Björnson wrote:
>> Hi.
>> I have a question regarding type classes and FunDeps.
>> Consider the following code :
>>
>>> class Class2 a b | a -> b
>>
>>> class IsFoo a
>>> data Bar a = Bar a
>>
>>> instance IsFoo a => Class2 a a
>>> instance IsFoo a => Class2 (Bar a) a
>>
>> The last two instantiations will yield a 'Functional dependencies
>> conflict error'. From what I understand, this is because (Bar a)
>> *MAY* instantiate the type class isFoo, which would obviously lead to
>> a conflict between the two instantiations.
>
> The typeclass IsFoo is irrelevant to the fundep.
> The problem arises here:
>
> class Class2 a b | a -> b
>
> data Bar a = Bar a
>
> instance Class2 a a
> instance Class2 (Bar a) a
>
> because a and Bar a could be instantiated to two different types eg
> Bool and Bar Bool, yet the result type would be the same, but the
> fundep specifies that the 'a' type uniquely determines the 'b' type,
> hence the conflict.

Please ignore the above "explanation" cause it's wrong ;-)
I must have had an attack of fundep dyslexia and read the arrow the wrong 
way...

Regards, Brian.