[Haskell-cafe] usage of traversal

[Michael Snoyman]

Forgot to CC the list:

I'd be -1 on an operator, I think having a named function for this is a
good thing for readability of code.

As far as good style: I personally think it is. In classy-prelude, I
actually export the Foldable-based `mapM` by default, and will regularly
use that (or forM) for this kind of code.

On Tue, Sep 23, 2014 at 8:27 AM, Kazu Yamamoto <kazu at iij.ad.jp> wrote:

> Hi,
>
> Recently, I need to write the following code:
>
>     getModTime :: Maybe FilePath -> IO (Maybe UTCTime)
>     getModTime mfile = case mfile of
>         Nothing -> return Nothing
>         Just file -> Just <$> getModificationTime file
>
> I feel that this is redundant. So, I used 'traverse' instead:
>
>     getModTime :: Maybe FilePath -> IO (Maybe UTCTime)
>     getModTime mfile = getModificationTime `traverse` mfile
>
> First, I would like to know whether or not this is a good coding style.
>
> Second, if this is acceptable, why don't we define an operator? For
> instance,
>
>    (<:>) :: (Traversable t, Applicative f) => (a -> f b) -> t a -> f (t b)
>    (<:>) = traverse
>
>     getModTime :: Maybe FilePath -> IO (Maybe UTCTime)
>     getModTime mfile = getModificationTime <:> mfile
>
> Is there such an operator already?
>
> Regards,
>
> --Kazu
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