Re: [Haskell-cafe] Re: All equations must have the same arity - why?

On 13 Jan 2008, at 5:27 PM, Achim Schneider wrote:

"Neil Mitchell" wrote:

...

Hi,

It's nice to write functions in point free style:

f = sort . nub

But sometimes I have to add an extra case, on a certain value:

f [] = [1] f = sort . nub

But now these equations have different arities, and its rejected by Haskell. Why does this not simply desugar to:

f [] = [1] f x = (sort . nub) x

i.e. lift the arities to the longest argument list.

Is there a reason this isn't done?

Answer #2:

Because you can't write

f x = case x of [] -> [1] -> sort.nub

But why not? Treating case as syntax sugar for a higher-order function, so that all binding occurrences of lambdas in the unsugared code are associated with lambdas, is a good thing, and offers a natural way of desugaring the above, one case at a time. jcc