d6y/IO.md

## IO.md

      
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              IO.md
            
          
    IO

Basic concepts and mechanisms


pure functions, no side effects, good for reasoning
IO system separates pure functions from side-effects
Hello world, various ways to run it
putStrLn and getLine, do notation
Types of IO () and IO String
Reverse example, with IF and recursive call
Values you can pass around, executed by main (or in the REPL, or by unsafePerformIO)

More functions (some via import Control.Monad)


putChar, putStr, print, getChar
when
sequence :: [IO a] -> IO [a]
mapM and mapM_ (map and then sequence)
forM
forever
getContents
interact

System functions

From import System.IO, and System.Directory


openFile, hGetContents, hClose, and withFile
hGetLine, hPutStr, hPutStrLn, hGetChar
readFile, writeFile, appendFile
doesFileExist
hSetBuffering, BufferMode (NoBuffering, LineBuffering, BlockBuffering (Maybe Int)), hFlush.
openTempFile, removeFile, renameFile

### From System.Environment

getProgName, getArgs

From System.Random

Run stack install random or cabal install random

random, randoms, randomR, randomRs
RandomGen, Random
StdGen, mkStdGen, getStdGen, newStdGen

E.g.,
random (mkStdGen 100) :: (Bool, StdGen)

ByteStrings


Data.ByteString (strict) and Data.ByteString.Lazy.
Word8 (range 0-255)
pack, unpack
cons (lazy), cons' (strict), empty
fromChunks, toChunks
functions with same name as those in List
functions with same name as System.IO

import qualified Data.ByteString.Lazy as B  
import qualified Data.ByteString as S 

IO Code can throw exceptions


as can pure code (such as div 4 0 or head [])
can be caught in IO only
catch (from import Control.Exception.Base)
IOError
isDoesNotExistError (in System.IO.Error), isAlreadyExistsError ...
userError, ioError

Exercises

1. Lottery

From: https://github.com/noelmarkham/learn-you-a-haskell-exercises/blob/master/09-input-and-output.hs
{- Write a lottery number picker
 - This function should take a StdGen instance, and produce a list of six unique numbers between 1 and 49, in numerical order
 -}
lottery :: StdGen -> [Int]
lottery gen = undefined

2. Xmas

Draw an ASCII xmas tree.
E.g., combine triangle(s) and rectangle.
The program should accept the height of the tree as input.
Inspiration: https://duckduckgo.com/?q=ascii+christmas+tree&t=ffab&iax=1&ia=images
E.g.,
How tall a tree? 3

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