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10-input-and-output.hs

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+
+main = putStrLn "hello, world"  -- putStrLn :: String -> IO()
+
+-- IO actions are performed when they are the result of `main`
+main = do
+  putStrLn "Hello, what's your name?"  :: IO ()
+  name <- getLine
+  putStrLn ("Hey " ++ name ++ "!") :: IO ()
+
+main = do
+    putStrLn "Enter a number:"
+    strResult <- getLine
+    let numResult=(read strResult) :: Int
+    putStrLn (show (numResult * 10))
+
+-- main is always oftype main :: IO a, but we normally don't specify the type explicitly
+
+-- Well, I/O actions will be performed if we type them out in GHCi...
+
+-- In do-blocks, we can use `let` without `in`:
+main = do
+  putStrLn "Name?"
+  name <- getLine
+  let bigName = map toUpper name
+  putStrLn $ "hey, " ++ bigName ++ "."
+
+-- Example that continually reads line and prints out reversed words:
+
+reverseWords :: String -> String
+reverseWords = unwords . map reverse . words
+
+main = do
+  line <- getLine
+  if null line
+  then return ()
+  else do
+    putStrLn $ reverseWords line
+    main
+
+-- putStr - returns IO that prints string to terminal, no \n
+-- putChar - returns IO that prints char to terminal
+-- print - for some value deriving Show, do putStrLn . show
+-- getChar - IO that reads character of input - because of buffering, read won't happen until user hits return
+-- when from Control.Monad - if value is True, return the IO passed in. Else, return ():
+
+import Control.Monad
+
+main = do
+  c <- getChar
+  when (c /= ' ') $ do
+    putChar c
+    main
+
+-- sequence - list of IO and returns IO that performs them in order - sequence :: [IO a] -> IO [a]
+-- mapM maps a function over the list then sequences it, mapM_ does the same and throws out the result
+-- forever - takes an IO and returns an IO that repeats the original action forever - from Control.Monad - e.g.
+
+import Control.Monad
+import Data.Char
+
+main = forever $ do
+  putStr "Input: "
+  l <- getLine
+  putStrLn $ map toUpper l
+
+-- Files & streams
+-- getContents reads from stdin in a lazy manner
+
+getContents :: IO String
+
+import Data.Char
+
+main = do
+  contents <- getContents
+  putStr (map toUpper contents)
+
+-- This reads from contents as needed, rather than all at once.
+-- Now, a program that takes input and prints lines shorter than 10 chars
+
+main = do
+  contents <- getContents
+  putStr (shortLinesOnly contents)
+
+shortLinesOnly :: String -> String
+shortLinesOnly input = unlines . (filter (\x -> (length x) < 10)) . lines input
+
+-- We can use `interact` to read contents, map them String -> String, then print the result:
+main = interact $ unlines . (filter ((<10) . length)) . lines
+
+-- File IO
+-- Here's an example reading a file:
+
+import System.IO
+main = do
+  handle <- openFile "file.txt" ReadMode
+  contents <- hGetContents handle
+  putStr contents
+  hClose handle
+
+-- Alternatively, we can use `withFile` to automatically close the Handle:
+main = do
+  withFile "file.txt" ReadMode (\handle -> do
+    contents <- hGetContents handle
+    putStr contents)
+
+-- We could implement withFile ourselves:
+withFile :: FilePath -> IOMode -> (Handle -> IO a) -> IO a
+withFile path mode fn = do
+  handle <- openFile path mode
+  result <- fn handle
+  hClose handle
+  return result
+
+-- hGetLine, hPutStr, hPutStrLn, hGetChar all exist for file IO
+-- readFile :: FilePath -> IO String can simplify the above - reads contents of file to IO String action
+-- writeFile :: FilePath -> String -> IO () - overwrites contents of file
+-- appendFile :: FilePath -> String -> IO () - appends to contents of file
+-- We can change the buffering mode using hSetBuffering and the BufferMode type
+-- Can also use hFlush to flush a handle (IO ())
+
+-- Program for removing line from todo.txt:
+
+import System.IO
+import System.Directory
+import Data.List
+
+main = do
+  handle <- openFile "todo.txt" ReadMode
+  (tempName, tempHandle) <- openTempFile "." "temp"
+  contents <- hGetContents handle
+
+  let todoTasks = lines contents
+      numberedTasks = zipWith (\n line -> show n ++ " - " ++ line) [0..] todoTasks
+
+  putStrLn "These are your TODO items:"
+  putStr $ unlines numberedTasks
+  putStrLn "Which one do you want to delete?"
+
+  numberString <- getLine
+  let number = read numberString
+      newTodoItems = delete (todoTasks !! number) todoTasks
+
+  hPutStr tempHandle $ unlines newTodoItems
+  hClose handle
+  hClose tempHandle
+  removeFile "todo.txt"
+  renameFile tempName "todo.txt"
+
+-- We can use getArgs :: IO [String] and getProgName :: IO String from System.Environment to get CLI args
+
+-- Let's build a simple CLI application:
+
+import System.Environment
+import System.Directory
+import System.IO
+import Data.List
+
+add :: [String] -> IO ()
+add [fileName, todoString] = appendFile fileName (todoString ++ "\n")
+
+view :: [String] -> IO ()
+view [fileName] = do
+  contents <- readFile fileName
+  let todoTasks = lines contents
+      numberedTasks = zipWith (\n line -> show n ++ " - " ++ line) [0..] todoTasks
+  putStr $ unlines numberedtasks
+
+-- ...similarly for `remove`
+
+dispatch :: [(String, [String] -> IO ())]
+dispatch = [
+    ("add", add),
+    ("view", view),
+    ("remove", remove),
+  ]
+
+main = do
+  (command:args) <- getArgs
+  let (Just action) = lookup command dispatch
+  action args
+
+-- Random number generation:
+import System.Random
+
+-- random :: (RandomGen g, Random a) -> g -> (a, g)
+-- RandomGen - a source of randomness
+-- Random - typeclass of things that can take on random values
+
+-- To manually create a random generator, use mkStdGen :: Int -> StdGen
+-- So, now we can do this:
+
+random (mkStdGen 100) :: (Int, StdGen)  -- this casts the a -> Int
+
+-- We can use the `randoms` function to get an infinite sequence of random values:
+take 5 $ randoms (mkStdGen 11) :: [Int]
+
+-- We can get random values in a range using randomR and randomRs
+-- randomR :: (RandomGen g, Random a) :: (a, a) -> g -> (a, g)
+
+-- But we may want to randomly initialize StdGen rather than providing a seed
+-- To do this, we can use `getStdGen :: IO StdGen`:
+
+main = do
+  gen <- getStdGen
+  putStr $ take 20 (randomRs ('a','z') gen)
+
+-- Bytestrings
+-- [Char] can be inefficient because Char is not a fixed width
+-- Instead, we can use bytestrings which are [Word8] mostly
+-- There are 2 kinds - Data.Bytestring (strict) and Data.Bytestring.Lazy (lazy)
+
+-- pack :: [Word8] -> ByteString
+-- Word8 is 0 thru 255. For example:
+
+Data.ByteString.pack [99,97,110]  -- == Chunk "can" Empty
+
+-- unpack :: ByteString -> [Word8]
+-- fromChunks takes a list of strict bytestrings and creates a combined lazy bytestring
+-- cons - adds a Word8 to the beginning of a bytestring, lazily
+-- cons' - if you are prepending lots of Word8, use the strict version since it's more efficient
+-- empty - makes an empty ByteString
+
+-- Example program copying one file to another, lazily. This already exists, but is a good example
+
+import System.Environment
+import qualified Data.ByteString.Lazy as B
+
+main = do
+  (fileName1:fileName2:_) <- getArgs
+  copyFile fileName1 fileName2
+
+copyFile :: FilePath -> FilePath -> IO ()
+copyFile src dst = do
+  contents <- B.readFile src
+  B.writeFile dst contents
+
+-- This lazily transfers the file in chunks
+
+-- In I/O context, we can actually catch exceptions arising from IO actions using
+-- `catch` from System.IO.Error:
+-- catch :: IO a -> (IOError -> IO a) -> IO a
+
+import System.Environment
+import System.IO
+import System.IO.Error
+
+main = toTry `catch` handler
+
+toTry :: IO ()
+toTry = do (fileName:_) <- getArgs
+  contents <- readFile fileName
+  putStrLn $ "The file has " ++ show (length (lines contents)) ++ " lines!"
+
+handler :: IOError -> IO ()
+handler e = putStrLn "Whoops, had some trouble!"
+
+-- We can use ioError to re-raise the IOError
+-- There are predicates over IOError to figure out what type of error it was:
+-- isAlreadyExistsError, isDoesNotExistError, isAlreadyInUseError, isFullError, isEOFError, isIllegalOperation, isPermissionError, isUserError
+
+-- isUserError is True when we use `userError` to create an IOError:
+ioError $ userError "Uh, oh!"
+
+

11-thinking-functionally.hs

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+
+