How to “debug” Haskell with printfs?

Question

trace is the easiest to use method for debugging. It’s not in IO exactly for the reason you pointed: no need to lift your code in the IO monad. It’s implemented like this

trace :: String -> a -> a
trace string expr = unsafePerformIO $ do
    putTraceMsg string
    return expr

So there is IO behind the scenes but unsafePerformIO is used to escape out of it. That’s a function which potentially breaks referential transparency which you can guess looking at its type IO a -> a and also its name.

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