Monads: bind/return

gistfile1.hs

-- Uso de "bind"

import Data.List.HT (chop)
import Control.Monad(liftM, foldM)

lineas :: IO ([[String]])
lineas = 
    (liftM -- (String -> [[String]]) -> (IO (String) -> IO ([[String]]))
        ((map (chop (== ','))).lines)) -- String -> [[String]]
        . readFile -- IO (String)
        $ "triangles.txt" 
    >>= 
    return -- [[String]] -> IO ([[String]]); en realidad así no hace absolutamente nada

-- lineas2 :: IO ([[String]])
-- lineas2 :: IO ([[Integer]])
lineas2 :: IO ([Integer])
lineas2 =
    readFile "triangles.txt" -- IO (String)
    >>=
    (\k -> return (lines k)) -- String -> IO ([String])
    >>=
    (\k -> return (map (chop (== ',')) k)) -- [String] -> IO ([[String]])
    >>=
    (\k -> return $ (map $ map $ (filter (/= '\r'))) k) -- [[String]] -> IO ([[String]])
    >>=
    (\k -> return $ (map $ map read) k) -- [[String]] -> IO ([[Integer]])
    >>=
    (\k -> return $ map sum k) -- [[Integer]] -> IO ([Integer])

-- lo mismo que lineas2, con composición para evitar lambda
lineas3 :: IO ([Integer])
lineas3 =
    readFile "triangles.txt" >>= 
    return.lines >>= 
    return.(map (chop (== ','))) >>= 
    return.(map $ map $ (filter (/= '\r'))) >>= 
    return.(map $ map read) >>= 
    return.(map sum)

-- todo junto
lineas4 :: IO ([Integer])
lineas4 =
    liftM ((map sum) .
           (map $ map read) . 
           (map $ map $ (filter (/= '\r'))) .
           (map (chop (== ','))) .
           lines) $ readFile "triangles.txt" 

operacion :: IO (Integer)
operacion = 
    return [1..10] -- IO ([Integer])
    >>= 
    (foldM (\ j k -> return (10*(j+k))) 1) -- [Integer] -> IO (Integer)

pares :: IO ([Integer])
pares = 
    return [1..10] -- IO ([Integer])
    >>= 
    (\k -> return (filter even k)) -- [Integer] -> IO ([Integer])

{- 

http://projecteuler.net/project/triangles.txt

% -> head triangles.txt 
-340,495,-153,-910,835,-947
-175,41,-421,-714,574,-645
-547,712,-352,579,951,-786
419,-864,-83,650,-399,171
-429,-89,-357,-930,296,-29
-734,-702,823,-745,-684,-62
-971,762,925,-776,-663,-157
162,570,628,485,-807,-896
641,91,-65,700,887,759
215,-496,46,-931,422,-30
-}