# [Haskell-cafe] Missing join and split

Sterling Clover s.clover at gmail.com
Mon Dec 31 01:05:50 EST 2007

So a simple thing occured to me today. Rather than worry about the
correct behavior for a join/split pair, we should just add
unintercalate to the library. A bit verbose as a name, but at least
there's no ambiguity.

--s

On Dec 29, 2007, at 2:18 PM, David Roundy wrote:

> On Fri, Dec 28, 2007 at 04:24:38PM +0100, Benja Fallenstein wrote:
>> On Dec 28, 2007 3:55 PM, David Roundy <daveroundy at gmail.com> wrote:
>>> On Dec 28, 2007 9:51 AM, Benja Fallenstein
>>> <benja.fallenstein at gmail.com> wrote:
>>>> 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.
>
>>> 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
> _______________________________________________