Haskell Language IO Writing to stdout


Per the Haskell 2010 Language Specification the following are standard IO functions available in Prelude, so no imports are required to use them.

putChar :: Char -> IO () - writes a char to stdout

Prelude> putChar 'a'
aPrelude>  -- Note, no new line

putStr :: String -> IO () - writes a String to stdout

Prelude> putStr "This is a string!"
This is a string!Prelude>  -- Note, no new line

putStrLn :: String -> IO () - writes a String to stdout and adds a new line

Prelude> putStrLn "Hi there, this is another String!"
Hi there, this is another String!

print :: Show a => a -> IO () - writes a an instance of Show to stdout

Prelude> print "hi"
Prelude> print 1
Prelude> print 'a'
Prelude> print (Just 'a')  -- Maybe is an instance of the `Show` type class
Just 'a'
Prelude> print Nothing

Recall that you can instantiate Show for your own types using deriving:

-- In ex.hs
data Person = Person { name :: String } deriving Show
main = print (Person "Alex")  -- Person is an instance of `Show`, thanks to `deriving`

Loading & running in GHCi:

Prelude> :load ex.hs
[1 of 1] Compiling ex             ( ex.hs, interpreted )
Ok, modules loaded: ex.
*Main> main  -- from ex.hs
Person {name = "Alex"}