putStrLn

putStrLn is a standard function in Haskell used to print a line of text to the standard output. It is an IO action that, when executed, outputs the given string followed by a newline character.

Type and behavior: putStrLn has the type signature putStrLn :: String -> IO (). It takes a String and returns an IO action that produces no value (unit) after performing the print. The function prints the string exactly as provided and appends a newline; it does not add quotes or escape characters. If you want to display other types, you must convert them to strings first, for example with show, or use print.

Relation to other IO operations: putStrLn is a convenience wrapper around putStr that adds a trailing newline. It is distinct from print, which has the type Show a => a -> IO () and prints values by converting them with show, including quotes for strings. Since putStrLn operates on String, it is often used for simple text output in scripts and examples.

Usage: putStrLn is commonly used in simple programs and examples to display messages to the user. Example: main = putStrLn "Hello, world!". For more interactive output, it can be combined with other IO actions, such as getLine, in a do block, e.g., main = do putStrLn "Enter your name:" name <- getLine; putStrLn ("Hello, " ++ name).

See also: putStr, print, getLine, unlines.