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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