Haskell for Perl ProgrammersBasic ExamplesLists

2.2. Lists

Reversing a List

myreverse :: [a] -> [a]

myreverse mylist = helper mylist [] where
    helper :: [a] -> [a] -> [a]
    -- A stopping condition
    helper [] as = as
    -- Recurse
    helper (b:bs) as = helper bs (b:as)

Run-Length Encoding

rle :: Eq a => [a] -> [(a,Integer)]

rle [] = []
rle (a:[]) = [(a,1)]
rle (x:xs) = (if (x == a)
              then (a,count+1):as
              else (x,1):(a,count):as
             ) where
        ((a,count):as) = (rle xs)

Quick-Sort

qsort []     = []
-- ++ is list concatenation
qsort (x:xs) = qsort elts_lt_x ++ [x] ++ qsort elts_greq_x
                 where
                   -- Choose the elements out of xs that are lesser than x
                   elts_lt_x   = [y | y <- xs, y < x]
                   elts_greq_x = [y | y <- xs, y >= x]

Written by Shlomi Fish