Haskell - Debug Print in if block

Question

I want to debug my program by printing something

For example,

isPos n 
    | n<0       = False
    | otherwise = True

I want something like:

isPos n 
    | n<0       = False AND print ("negative")
    | otherwise = True  AND print ("positive")

Is it possible to do in Haskell?

Read before haskell.org/haskellwiki/Debugging Read after stackoverflow.com/questions/672979/… — Jonke
– Jonke, Commented Sep 24, 2012 at 6:07

beta · Accepted Answer · 2012-09-23 12:17:45Z

23

As hammar said, use trace from the Debug.Trace module. A tip I have found useful is to define the function debug:

debug = flip trace

You could then do

isPos n
  | n < 0     = False `debug` "negative"
  | otherwise = True  `debug` "positive"

The benefit of this is that it is easy to enable/disable the debug printing during development. To remove the debug printing, simply comment out rest of the line:

isPos n
  | n < 0     = False -- `debug` "negative"
  | otherwise = True  -- `debug` "positive"

answered Sep 23, 2012 at 12:17

beta

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3 Comments

gspr Over a year ago

I've often also found the variation debug = (flip trace) False useful. If I have a guards-based function definition, I simply insert a (or multiple) | debug ("Interesting value is " ++ show whatever) = undefined as the first guard(s). The trace is always run, but since debug x is always False, the guard always fails and the program behaves as normal. This makes it easy to quickly inspect values, at least in functions that already have guards.

Achal Dave Over a year ago

You can also have the following two definitions at the top of your file, and simply toggle between them to toggle debug mode: debug = flip trace and debug a b = a

Achal Dave Over a year ago

Apparently I can't edit that comment anymore; a simpler way is to have a doDebug boolean and define debug as debug a b = if doDebug then trace b a else a

hammar · Accepted Answer · 2012-09-23 06:50:03Z

20

Use Debug.Trace.trace.

import Debug.Trace

isPos n 
  | n < 0     = trace "negative" False
  | otherwise = trace "positive" True

answered Sep 23, 2012 at 6:50

hammar

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