On Tue, Jan 20, 2009 at 11:10 PM, wren ng thornton <wren@freegeek.org> wrote:

Andrew Wagner wrote:

Strange little bit of code:
http://moonpatio.com:8080/fastcgi/hpaste.fcgi/view?id=829#a829

If I do any of the following, all of which seem natural to me, it fails to
typecheck:

1. move f out of the 'where' clause (with or without a type signature)
2. put the same type signature on f as is on (/\)
3. replace f with (/\) completely

What's going on here?

> :t (nub .) . (++)
(nub .) . (++) :: (Eq a) => [a] -> [a] -> [a]

> :t foldr (map . (nub .) . (++))
foldr (map . (nub .) . (++)) :: (Eq a) => [[a]] -> [[a]] -> [[a]]

The type you give to (/\) is more restrictive than the type of the expression, and f uses the generality of the expression.

--
Live well,
~wren
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