Until we get rid of (>>) from Control.Monad and we can then rely upon (*>)
and (>>) having the same performance characteristics mapM_ and traverse_
are not interchangeable. I realize you didn't say anything about those
functions, but hear me out.
The tentative migration plan is, assuming the "monad of no return" proposal
goes through, to first eliminate (>>) then fix mapM_. This avoids the very
awkward intermediate state that would be a result of directly implementing
your proposal here where mapM_ needs a stronger constraint (Monad) than
mapM does, which would use Applicative!
I therefore very strongly advocate waiting for the elimination of (>>)
through the monad of no return proposal before we proceed with any
simplification of Traversable.
-Edward
On Sat, Sep 26, 2015 at 5:05 PM, David Feuer
Traversable currently has traverse, sequenceA, sequence, and mapM methods. sequence and mapM seem to be dinosaurs now. I'd like to suggest that sequenceA be turned into a deprecated function and mapM be moved from Control.Monad to Data.Traversable and re-exported from Control.Monad.
I expect most instance declarations to survive unchanged. The only CPP pain point will be that instances that define sequenceA will have to define sequence instead.
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