[Haskell-cafe] List fusion of nested lists

[Joachim Breitner]

Dear Cafe,

I’m trying to exploit list fusion as provided by GHC (build/foldr). One
function that I want to get fusable is this, it splits a list of
integeres into maximal monotonous subsequences:

streaks :: [Integer] -> [[Integer]]
streaks [] = []
streaks (x:xs) = let (this,rest) = oneStreak (x:xs)
                 in this:streaks rest

oneStreak :: [Integer] -> ([Integer], [Integer])
oneStreak [x] = ([x],[])
oneStreak l@(x:y:_) = splitWhile2 (\a b -> a `compare` b == x `compare` y) l

splitWhile2 :: (Integer -> Integer -> Bool) -> [Integer] -> ([Integer], [Integer])
splitWhile2 p [x] = ([x],[])
splitWhile2 p (x:y:xs) | p x y = let (s,r) = splitWhile2 p (y:xs) in (x:s,r)
                       | otherwise = ([x],y:xs) 


Now I’d like to implement streaks in terms of build and foldr such that
it is subject to list fusion. Especially, when used in
        concatMap (streaks . func) :: [X] -> [[Integer]]
where
        func :: X -> [Integer]
is implemented with buildr, this should ideally remove all intermediate
lists.

Can this be done with list fusion at all? How would I go about it?

If the above example is too complicated, known how it would work for
Data.List.group would help me already a lot.

Greetings,
Joachim


-- 
Joachim "nomeata" Breitner
  mail at joachim-breitner.de  |  nomeata at debian.org  |  GPG: 0x4743206C
  xmpp: nomeata at joachim-breitner.de | http://www.joachim-breitner.de/

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