Fumiaki Kinoshita fumiexcel at
Tue Mar 31 14:11:40 UTC 2015

I understand the ground. It seems reasonable not to add symbols facilely.

But in this case, the "too specialized" version is exported while more
fundamental one is not.
Although folks (including me) use mapM_ mostly today, someday we will like
to have traverse_, I guess.

2015-03-31 19:41 GMT+09:00 Edward Kmett <ekmett at>:

> We deliberately took no more symbols than we needed in 7.10 from Prelude
> as part of the Foldable/Traversable Proposal. There are multiple
> combinators in Data.Foldable and Data.Traversable that we do not export.
> traverse_ is one of them as, strictly speaking, traverse_ was a symbol we
> didn't have to take.
> If we had would anybody have complained any more loudly? Not sure... but
> it was a deliberate choice to not bring in any symbols into Prelude that
> weren't already there that weren't part of the definition of a class or
> needed to define instances that already existed.
> -Edward
> On Mon, Mar 30, 2015 at 11:33 PM, Fumiaki Kinoshita <fumiexcel at>
> wrote:
>> Well, I see. It'd be nice.
>> That aside, the absence of traverse_ doesn't seem to be intended (even
>> the documentation for mapM_ says "mapM_ is just traverse_"!)
>> 2015-03-30 16:54 GMT+09:00 Herbert Valerio Riedel <hvr at>:
>>> On 2015-03-30 at 07:05:56 +0200, Fumiaki Kinoshita wrote:
>>> [...]
>>> > I found out that (<>) (in Data.Monoid) is missing, also. It would be
>>> nice
>>> > to reexamine Prelude to export things we want to export.
>>> Fwiw, (<>) was actually left-out as it wasn't required (it's just a an
>>> alias for `mappend`), *and* to keep our options open (or at least not
>>> make it more difficult) in terms of possible migration-plans available
>>> for the case we'd be moving 'Semigroup' to base/Prelude at some point in
>>> the future.
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