Proposal: Debug.Trace.traceWith

[Kris Nuttycombe]

On Sat, Feb 10, 2018 at 6:48 AM, Michael Sloan <mgsloan at gmail.com> 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 <david.feuer at gmail.com> 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" <kris.nuttycombe at gmail.com>
> 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
>
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