Haskell
Chapter 9
More Input and More Output
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Files and Streams
Transforming Input
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Not covered
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brackets
command-line arguments
bytestrings
Transforming input
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Common pattern: get string from input, transform, output
result
Use interact
main = interact shortLinesOnly
shortLinesOnly :: String -> String
shortLinesOnly = unlines . filter (\line -> length line < 10) . lines
Quick Ex: Look up lines, unlines
Another example
respondPalindromes :: String -> String
respondPalindromes =
unlines .
map (\xs -> if isPal xs then "palindrome" else "not
a palindrome") .
lines
isPal :: String -> Bool
isPal xs = xs == reverse xs
main2 = interact respondPalindromes
Reading a file
import System.IO
main3 = do
handle <- openFile "haiku.txt" ReadMode
contents <- hGetContents handle
putStr contents
putStr "\n"
hClose handle
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openFile :: FilePath -> IOMode -> IO Handle
type FilePath = String
data IOMode = ReadMode | WriteMode | AppendMode |
ReadWriteMode
Reading a file – simpler
import System.IO
main4 = do
contents <- readFile "haiku.txt"
putStr contents
putStr "\n"
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readFile :: FilePath -> String -> IO()
Randomness
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Referential transparency: function given the same
parameters twice must return same result
SO, we bring in randomness from outside (like using Unix
time stamp for a seed)
Take a random generator, return a random value and new
random generator
random : : (RandomGen g, Random a) => g -> (a, g)
Take an integer, return a random generator
mkStdGen :: Int -> StdGen
* more random functions in book
Example
import System.Random
threeCoins :: StdGen -> (Bool, Bool, Bool)
threeCoins gen =
let
(firstCoin, newGen) = random gen
(secondCoin, newGen') = random newGen
(thirdCoin, newGen'') = random newGen'
in (firstCoin, secondCoin, thirdCoin)
--threeCoins (mkStdGen 22)