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