debugging the div' function using guards

div_prime.hs

import Debug.Trace

div' :: Integer -> Integer -> Integer
div' x y
  | trace ("x is: " ++ (show x) ++ " and y is: " ++ (show y)) False = undefined
  | otherwise = x `div` y
  
-- guards are evaluated from top to bottom until a match is found, so we fire off
-- our call to trace (which returns False) and move on to the next guard (which is
-- our real implementation). 

Can't you also just do

div' x y = "blah" `trace` div x y

?

Another technique is to use a where clause to simplify creation of the message:

div' x y | trace msg False = undefined where msg = ...
div' x y = ... original definition...

To disable tracing just comment out the first line. Also have a look at Debug.NoTrace to globally disable tracing.

@erantapaa I really enjoy that syntax. Thanks for the tip!