[Haskell-cafe] Re: Why is there no splitBy in the list module?

[Jon Fairbairn]

On 2006-07-20 at 18:31BST I wrote:
> On 2006-07-13 at 10:16BST I wrote:
> > Hooray!  I've been waiting to ask "Why aren't we asking what
> > laws hold for these operations?"
> 
> Having thought about this for a bit, I've come up with the
> below. This is intended to give the general idea -- it's not
> polished code,

And already some changes (but all to comments) I'd
appreciate comments on the law below.

--- lib/split.lhs       2006/07/21 08:54:28     1.1
+++ lib/split.lhs       2006/07/21 09:42:49
@@ -17,10 +17,17 @@

   => "A Random List Of Words\nOn Lines"

+  I think the relevant law is
+  (forall x. all (not . p) (map f x)) =>
+  segmentsSatisfying (not . p) . fromParts . mapRight f . parts p
+  == segmentsSatisfying (not . p)
+
+  In other words, if f doesn't add any "not . p" elements,
+  the segments satisfying "not . p" are unchanged.

 > contiguousParts p l = [a | Right a <- parts p l]

-  so words = contiguousParts Char.isAlphaNum
+  so words = contiguousParts (not . Char.isSpace)

 > segmentsSatisfying predicate
 >     = concat . map dropSeps . parts predicate

-- 
Jón Fairbairn                              Jon.Fairbairn at cl.cam.ac.uk