'Nice' Debugging?

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#1

Are there any nice ways to use Debug.Trace functions?

(it’s a bit of an oxymoron, I know!)

Small problem: if I insert them into my code, and want to uncomment them I usually also have to worry about whitespace again (maybe an auto formatter is a good idea).

Quite happy I have gone a long time without them, but they are handy also.

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#2

One trick I learned a while ago is to use ' to differentiate between traced function and untraced one. Say i have foo with some args and I want to debug it. I will change code from

foo ... = ... foo ... -- first pattern match, recursive call
foo ... = ...         -- another pattern match

to

foo ... = traceShow (..., ..., ...) $ foo' ...  -- put the arguments in the tuple and then call the original function
foo' ... = ... foo ... -- first pattern match, recursive call
foo' ... = ...         -- another pattern match

This way, I don’t need to change te rest of the code and can quickly remove tracing by commenting the first line and removing the 's

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#3

One trick I use often is to add a bogus guarded case to functions:

fib n | trace ("fib " ++ show n) False = undefined
fib 0 = 0
fib 1 = 1
fib n = fib (n - 1) + fib (n - 2)
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#4

Usually, I manage to make trace a full line, so I just have to comment the line.
Do you have any example ?

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#5

Yes

foo = <lots of code>
    where !() = trace ("Something: " ++ show importantVariable) ()

(Always use where !() not where !_ otherwise you risk writing trace s instead of trace s () and the former doesn’t produce any output!)

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#6

There’s fancy wrappers in Hledger.Utils.Debug.

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#7

Albert Lai has a page on Debug.Trace which includes the following idiom:

If I am tired of writing “traceShow n” 3 times (and later deleting them thoroughly), here is a cool trick:

import Debug.Trace

f n | traceShow n False = undefined
f n | n <= 1 = n
    | r == 0 = f q
    | otherwise = f (3*n + 1)
  where
    (q, r) = n `divMod` 2

Explanation: A new case that the computer must check first, which will be summarily rejected anyway, but it’s the journey of printing n that matters.

Benefits: easy and unintrusive to add, and easy to delete later.

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#8

I’m not sure it counts as nice but I often add debug and performance stuff in as (the lazy) StateT, and then you can flip between evalStateT for non-debug and runStateT for debug, without having to erase the debug code.

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#9

Could you give an example ?

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#10

PerfT is used for performance measurement, but you could generalize the idea.

https://hackage.haskell.org/package/perf-0.12.0.0/docs/Perf-Types.html

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#11

This is better than my trick, one single line to comment out to disable tracing instead of also needing to remove 's. Thanks

#12

I’m new to Haskell coming from Python where I use print debugging frequently. I’m glad I found this page because there are some excellent suggestions here!

I often want to add a label to a traced value. Currently, I’m using this function:

debug :: Show a => String -> a -> a
debug label expr = trace (label ++ " " ++ show expr) expr

which can be used inline like this:

fac :: (Show t, Eq t, Num t) => t -> t
fac 0 = 1
fac n = debug "n" n * fac (n-1)

I really wish this function was defined in Debug.Trace.

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#13

I think I’d suggest instead

{-# LANGUAGE BangPatterns #-}

fac :: (Show t, Eq t, Num t) => t -> t
fac 0 = 1
fac n = n * fac (n-1)
  where !() = traceShow ("n", n) ()

This has at least two benefits over your debug: it doesn’t require defining a new debug combinator, and it’s a single line that can be added or deleted freely without changing the actual functional code.

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#14

As @tomjaguarpaw suggested you can use traceShow and tuples, so you can do things like

fac n = traceShow("n", n, "result", r) where r =
           n * fact ( n - 1)

I put two lines so that you can easily delete traceShow ... r = later on.

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