Re: [Haskell-cafe] Missing join and split

On Fri, Dec 28, 2007 at 04:24:38PM +0100, Benja Fallenstein wrote:

On Dec 28, 2007 3:55 PM, David Roundy wrote:

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On Dec 28, 2007 9:51 AM, Benja Fallenstein wrote:

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If you use intercalate to join, I would presume that you would want to use an inverse of it to split. I'd write it like this:

Of course, there is no inverse to intercalate

Right; I misspoke. What I meant was that you would want a split such that

intercalate a (split a xs) = a

for finite, total (a,xs) (and, since it's achievable, even for infinite xs). Of course, (split a xs = [xs]) satisfies that, but if we add the requirement that split is also supposed to do its job :-) then I think split is fully specified except for whether (split a [] = []) or (split a [] = [[]]). The latter seems better to me; e.g., it satisfies

split a (x ++ a ++ y) = split a x ++ split a y

Yes, the latter is what darcs' linesPS does.

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so if you want to use a "logical" approach, perhaps you'd want to define split first, and then define your join as the inverse of split.

If your join comes out as being intercalate, I suppose it's six of one, half a dozen of the other :-)

Well, your intercalate "\n" is not the same as "unlines" and the inverse of intercalate "\n" is not the same as lines, nor is its inverse (with " ") the same as words. It is true that intercalate " " is the same as unwords, however. So it does seem like the prelude doesn't really give us any hints as to what would be a useful generic join/split pair. -- David Roundy Department of Physics Oregon State University