Re: [Haskell-cafe] different arities
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.
Haskell (http://www.haskell.org/onlinereport/decls.html). The use of "arity" [..] 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).
The two reasons that occur to me are: (i) to catch more errors, specifically, to catch the error of accidentally missing an argument. Consider: f False False = 0 f False True = 1 f True = error "TF not allowed" f True True = 3 Without the rule, this would be accepted; with the rule, it is rejected - rightly so, I think. (ii) to make the meaning of pattern-matching easier to define and more regular. The current definition (sec 4.4.3.1 of the report - see the Translation sidebar) makes essential use of the fact that the array of patterns is rectangular. Without this, the precise order of matching might be ambiguous, or at least harder to define. http://www.haskell.org/onlinereport/decls.html#sect4.4.3.1 HTH. --KW 8-)
participants (2)
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Henning Sato Rosen -
Keith Wansbrough