Cookbooks
Haskell
Chapter 9. Testing and Debugging
Printing debug output

Haskell:
Printing debug output

How to:

A straightforward way to print debug information in Haskell is with the print function, which takes a value that’s an instance of the Show typeclass and outputs it to the console.

main :: IO ()
main = do
  let number = 42
  print number
  putStrLn "Debugging is a piece of cake in Haskell!"

-- Output:
-- 42
-- Debugging is a piece of cake in Haskell!

For more complex data structures, ensure they derive Show to enable pretty printing:

data Cake = Chocolate | Vanilla deriving Show

debugFlavor :: Cake -> IO ()
debugFlavor flavor = print flavor

main :: IO ()
main = debugFlavor Chocolate

-- Output:
-- Chocolate

Sometimes we want temporary debugging that’s easy to remove later. Enter the Debug.Trace module.

import Debug.Trace (trace)

main :: IO ()
main = putStrLn $ trace "This will print first" "This will print second"

-- Output:
-- This will print first
-- This will print second

The trace function prints the string when the value is evaluated, but it’s a side effect in the pure part of the code. It’s handy but use with caution!

Deep Dive

In ye olden days, debugging might’ve been the ol’ “print statement” trick. Haskell offers this with a functional twist and tools for cleaner debug practices. Enter print and the Debug.Trace module, as explored earlier.

Alternates to print include putStrLn for strings and putStr, if you’re not into that automatic newline. Debug.Trace also has variants like traceShow that work directly with Show instances, saving you a show call.

As for implementation details, print is essentially putStrLn . show. It prints any Show-able data to stdout. Debug.Trace functions, on the other hand, are meant for temporary use during development. They sneak into pure code and violate referential transparency which is a no-no in the long run.

Don’t forget logging libraries for serious applications, which offer more control and less “debug by print.”

See Also

Using an interactive shell (REPL)