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

Daniel Fischer daniel.is.fischer at web.de
Fri Feb 8 17:23:39 EST 2008

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

    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

More information about the Haskell-Cafe mailing list