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
m...@joachim-breitner.de | nome...@debian.org | GPG: 0x4743206C
xmpp: nome...@joachim-breitner.de | http://www.joachim-breitner.de/
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