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In and output

When initialising records for example, you might have noticed that you are unable to print them easily in GHCI. There is a simple solution to that. Derive Show so Haskell will know how to actually print your values:

data Dog = Dog
  { name :: String,
    race :: String,
    age :: Int
  }
  deriving (Show)

woody = Dog {name = "Woody", race = "Labrador", age = 12}

Then, in GHCI just type "woody" and the record will be printed.

Here is how we can read input from the command line and output it directly:

main = do
  putStrLn "Enter something: "
  userInput <- getLine
  putStrLn ("You entered: " ++ userInput)

Apart from putStrLn, which prints a string, followed by a new line, there are other functions for outputting:

  • putStr which does the same as putStrLn but doesn't create a new line
  • print which can take a value of any type, which is an instance of Show

mapM​

You might have wondered how to combine map & print. Hint, it does not work like this:

map print [1, 2, 3]

This code will create a list of IO functions, but will not print 1, 2 and 3 separately to the console.

To solves this problem, mapM exists. It takes a function and a list, and will map the function over the list in sequence.

mapM print [1, 2, 3]
1
2
3

forever​

forever will forever repeat the I/O action

import Control.Monad

main =
  forever
    ( do
        putStr "Enter something"
        input <- getLine
        putStrLn input
    )

getContents for input streams​

main = do
  pipeInput <- getContents
  putStr pipeInput

getContents expects us to provide an input stream, for example, via a UNIX pipe. For example, using cat:

cat somefile.txt | runhaskell main.hs

Our Haskell code will print what stands inside our text file.

Reading files​

import System.IO

main = do
  fileHandle <- openFile "text.txt" ReadMode
  fileContent <- hGetContents fileHandle
  putStr fileContent
  hClose fileHandle

This will print all the content in the given file.