Haskell Language IO Writing to stdout
Example
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"
"hi"
Prelude> print 1
1
Prelude> print 'a'
'a'
Prelude> print (Just 'a') -- Maybe is an instance of the `Show` type class
Just 'a'
Prelude> print Nothing
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"}
*Main>
