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 <[EMAIL PROTECTED]> wrote:
On Dec 28, 2007 9:51 AM, Benja Fallenstein
<[EMAIL PROTECTED]> 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
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