duncan.coutts:
On Wed, 2007-08-29 at 22:02 +0200, Henning Thielemann wrote:
I was used to use groupBy to split a list before every element that satisfies a predicate. E.g. splitting before every capital
Prelude> groupBy (\_ c -> not $ Char.isUpper c) "Hello World" ["Hello ","World"]
I also wanted to use this for splitting after specific elements. But this fails. I get
Prelude> groupBy (\c _ -> c /= ',') "1, 2, 3, 4" ["1, 2, 3, 4"]
but I wanted ["1,", " 2,", " 3,", " 4"]
I assumed that 'groupBy' would compare adjacent elements, but it seems to compare the leading element of each block with subsequent elements. Since the documentation doesn't tell which elements are actually compared it seems to assume that the comparison is commutative and transitive. Maybe this point should be made clearer. Additionally I think that comparing neighbouring elements is a useful behaviour and I suggest an according variant of 'groupBy' for Data.List. Opinions?
I noticed this interesting behaviour when implementing groupBy for lazy bytestrings. My QC tests showed I had the wrong answer for non-transative predicates. It was actually a lot harder to implement the H98 behaviour than the behaviour where we compare adjacent elements. So purely from an implementation point of view I'd be happy to change the behaviour to that which you suggest.
I'd second this. groupBy's exact behaviour was really fiddly to nail down in QC. H' should come with QuickCheck properties relating list functions to each other. -- Don