silently

Silently is a package that allows you to run an IO action and prevent it from writing to stdout, or any other handle, by using silence. Or you can capture the output for yourself using capture.

For example, the program

 import System.IO.Silently

 main = do
   putStr "putStrLn: " >> putStrLn "puppies!"
   putStr "silenced: " >> silence (putStrLn "kittens!")
   putStrLn ""
   (captured, result) <- capture (putStr "wookies!" >> return 123)
   putStr "captured: " >> putStrLn captured
   putStr "returned: " >> putStrLn (show result)

will print:

 putStrLn: puppies!
 silenced:
 captured: wookies!
 returned: 123

Limitations

Capturing/silencing might not work as expected if the action uses the FFI or conceals output under unsafePerformIO or similar unsafe operations.

Examples:

• FFI: https://github.com/hspec/silently/issues/3
• unsafePerformIO: https://github.com/bos/filemanip/issues/22