Relax Monad constraint in traceM and traceShowM to Applicative

Now that Applicative is a superclass of Monad in GHC 7.10, the current type signatures of traceM and traceShowM from Debug.Trace are too constricted:

traceM :: Monad m => String -> m ()
traceM string = trace string $ return ()

traceShowM :: (Show a, Monad m) => a -> m ()
traceShowM = traceM . show

These can be equivalently defined with Applicative:

traceM :: Applicative m => String -> m ()
traceM string = trace string $ pure ()

traceShowM :: (Show a, Applicative m) => a -> m ()
traceShowM = traceM . show

However, the M suffixes probably won't make sense after changing the type signatures, so it might be wise to create new traceA and traceShowA functions and deprecate traceM and traceShowM.

Trac metadata

Trac field	Value
Version	7.10.1-rc1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ekmett, hvr
Operating system
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changed weight to 5

added Tfeature request Trac import core libraries labels

We've adopted the practice of generalizing the existing fooM Combinators in base where possible as part of 7.10 but not giving a new set of fooA names to avoid namespace pollution, needless constraints and a complex deprecation cycle. It seemed the lesser of evils was to generalize the M suffix than ask literally everyone to retool.

I can live without the names traceA/traceShowA, but it would be nice to generalize the type signatures in the same vein as when/unless/guard.

No problem here with generalizing the types.

For Newcomers: easy ticket. Also add an entry in libraries/base/changelog.md, and add hvr as a reviewer.

added newcomer label

assigned to @RyanGlScott

Trac metadata

Trac field	Value
Differential revisions	→ D1029

mentioned in commit 39d83f23

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

changed milestone to %8.0.1

Milestone renamed

added Pnormal label