You have met up with one of the rules for function bindings in standard Haskell (http://www.haskell.org/onlinereport/decls.html). The use of "arity" in the error message is a poor choice of words. The rule is that each clause in the function binding must contain the same number of argument patterns.
Oh, I see, thanks Scott and Marco for answering! I am beginning to realise that everything worth knowing about Haskell really is in the report:-) Regards/Henning -------------------------------------------------------------------------------------------------- BTW, what might the reason for that be, is it for the sake of ease of translation/implementation, or for the sake of some principle? I mean, as soon as a person realises that there are multiple syntaxes for declaring a function, he/she will unify them to one abstract function in the head (where the resulting type matters, not the syntax). So, even if one probably should avoid mixing as in the example, for readability reasons, one might still argue that the restriction goes against PoLS. Just a thought.