Re: Proposal: Debug.Trace.traceWith

On Sat, Feb 10, 2018 at 6:48 AM, Michael Sloan wrote:

I often define

tracer prefix x = trace (prefix ++ ": " ++ show x) x

I also have `traceWithPrefix :: (Show b) -> String -> (a -> b) -> a -> a` defined, for the same reason, so I absolutely support adding that as well.

A bit arbitrary with the choice of ": ", but quite convenient. The with variant would be

tracerWith prefix f x = trace (prefix ++ ": " ++ show (f x)) x

The definition I use is simply traceWithPrefix p f a = trace (p ++ show (f a)) a On Fri, Feb 9, 2018 at 7:07 PM, David Feuer wrote:

Other another bike-shed color:

traceWith :: (a -> String) -> a -> a traceWith f a = trace (f a) a

traceShowWith :: Show b => (a -> b) -> a -> a traceShowWith f = traceWith (show . f)

Is there some situation where the `Show String` instance wouldn't be in scope? If `b` is `String` it works out just fine, so the first definition you gave here seems superfluous.

But on the other hand, pointfree.io tells me that you can write

trace =<< f trace =<< show . f

That's... remarkably clever, so clever that it never occurred to me in 4 years of writing Haskell. :) I see your point, but jumping to the `Monad Reader` instance is a bit surprising. At very least, maybe this approach ought to be documented in the module. Thanks for pointing it out! So, a question for all: if I were to want to create a pull request to add these functions (traceWith and traceWithPrefix) where would I go to do so? Thanks, Kris

On Feb 9, 2018 5:09 PM, "Kris Nuttycombe" wrote:

When using Debug.Trace for debugging, a very common operation is to prefer to trace only some subset or function of an intermediate result. As a consequence, I propose adding the following function to Debug.Trace:

traceWith :: (Show b) => (a -> b) -> a -> a traceWith f a = trace (show $ f a) a

While it's trivial to define, I have found this to be one of the most useful functions in my trace-based debugging toolkit, hence my proposal that it be added to base. It generalizes `traceShowId` in a meaningful and useful fashion.

As this is the first such proposal I've put forth, please let me know if I'm doing anything wrong with it!

Thanks,

Kris Nuttycombe

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries