Haskell - how to debug programs

[Salil Wadnerkar]

Normally, in any program, if it is not working, I would add some debug statements to understand why I am not getting the expected result. Since debug statements are not allowed in a pure function, how the expert Haskell programmers debug their programmers?

[Tim Holloway]

Salil Wadnerkar wrote:Normally, in any program, if it is not working, I would add some debug statements to understand why I am not getting the expected result. Since debug statements are not allowed in a pure function, how the expert Haskell programmers debug their programmers?

With a debugger. Just like most other languages. See https://stackoverflow.com/questions/6724434/how-to-debug-haskell-code

Debug statements aren't that common in languages like Java these days except for the most trivial stuff. It takes too long to stuff in the extra code and there's always a chance that you can accidentally introduce new bugs when you do so - especially if you don't freely bracket conditional constructs.

Instead we use the interactive debugger in our IDE. And/or add permanent logging statements that can be switched on and off without having to change the source code.

[Stephan van Hulst]

As much as Haskell fans like to spout that Haskell is 100% pure, it's not.

You can use Debug.Trace.trace to write strings to output, without changing your code too much.

[Vitaly Bragilevsky]

I'm next to suggest using the Debug.Trace module in those cases when you do need to debug your code by printing intermediate values. But I wouldn't say that there is a necessity to do that often. Using some sort of testing and calling your pure functions from GHCi may alleviate the need for conventional debugging in most cases. The GHCi debugger (with breakpoints and single-step execution) can also be used though I never found it really usable.