[Haskell-cafe] Re: problem with collection (container) class

[Daniel Fischer]

Am Freitag, 8. Februar 2008 22:14 schrieb Ben Franksen:
> If it's a bug then it is probably in 6.6.1 too, it just gets hidden by the
> fact that in 6.6.1 the -fglasgow-exts extensions cannot be activated
> separately. If you enable one of them, you get them all.
>
Thanks for the info, didn't know that. 

The problem was the error message, which didn't mention that each type 
variable may appear only once in an instance head, which I had temporarily 
forgotten. Then the message

Leandro.hs:32:0:
    Illegal instance declaration for `Container (Abb a b) a b'
        (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 `Container (Abb a b) a b'

looks rather confusing :)

> Cheers
> Ben
>
Cheers,
Daniel