Haskell

1
Haskell

• Chapter 9

2
More Input and More Output

• Files and Streams
• Transforming Input
• Not covered
• brackets
• command-line arguments
• bytestrings

3
Transforming input

• Common pattern get string from input, transform,
  output result
• Use interact
• main interact shortLinesOnly
• shortLinesOnly String -gt String
• shortLinesOnly unlines . filter (\line -gt
  length line lt 10) . lines

Preludegt lines "aa\nbb\nbb" "aa","bb","bb"
4
Some details

• interact String -gt String takes a string
  containing all the input and returns a string
  containing all the output.
• This is why we use lines/unlines with interact
• The reason you see output after pressing enter
  with interact (map toUpper) is because map
  toUpper acts lazily -- it can start giving output
  before all the input is known.
• In our example the output is from filter but
  same idea

http//stackoverflow.com/questions/16799755/haskel
l-interact-function
5
Another example

• respondPalindromes String -gt String
• respondPalindromes
• unlines .
• map (\xs -gt if isPal xs then "palindrome" else
  "not a palindrome") .
• lines
• isPal String -gt Bool
• isPal xs xs reverse xs
• main2 interact respondPalindromes

6
Reading a file

• import System.IO
• main3 do
• handle lt- openFile "haiku.txt" ReadMode
• contents lt- hGetContents handle
• putStr contents
• putStr "\n"
• hClose handle
• openFile FilePath -gt IOMode -gt IO Handle
• type FilePath String
• data IOMode ReadMode WriteMode AppendMode
  ReadWriteMode

7
Reading a file simpler

• Haiku (from wikipedia)
• Japanese poetry
• Essence is cutting
• Traditional has 17 on (syllables) in 3 phrases,
  with 5 7 5 syllables/phrase

• import System.IO
• main4 do
• contents lt- readFile "haiku.txt"
• putStr contents
• putStr "\n"
• readFile FilePath -gt String -gt IO()

8
Randomness

• Referential transparency function given the same
  parameters twice must return same result
• SO, we bring in randomness from outside (kind of
  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) gt g -gt (a, g)
• Take an integer, return a random generator
• mkStdGen Int -gt StdGen

more random functions in book
9
Example

• import System.Random
• threeCoins StdGen -gt (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)