Desugaring do-notation to Applicative

[Edward Kmett]

That is admittedly a pretty convincing example that we may want to provide
either a LANGUAGE pragma or a different syntax to opt in.

As a data point in this space, the version of the code I have in scheme
calls the version of 'do' that permits applicative desugaring 'ado'. A port
of it to Haskell with minor infelicities as a quasi-quoter can be found
here:

http://hackage.haskell.org/package/applicative-quoters-0.1.0.7/docs/Control-Applicative-QQ-ADo.html

However, that version uses an awkward hack to permit pattern matches to
fail.

-Edward


On Wed, Oct 2, 2013 at 12:24 PM, Reid Barton <rwbarton at gmail.com> wrote:

> On Wed, Oct 2, 2013 at 12:01 PM, Dag Odenhall <dag.odenhall at gmail.com>wrote:
>
>> What about MonadComprehensions, by the way? The way I see it, it's an
>> even better fit for Applicative because the return is implicit.
>>
> Yes, or ordinary list comprehensions for that matter.
>
> But there is a danger in desugaring to Applicative: it may introduce too
> much sharing. Currently a program like "main = print $ length [ (x, y) | x
> <- [1..3], y <- [1..10000000] ]" (or the equivalent in do-notation) runs in
> constant space with either -O0 or -O -fno-full-laziness. If you desugar it
> to a form like "main = print $ length $ (,) <$> [1..3] <*> [1..10000000]",
> then no optimization flags will save you from a space leak.
>
> It might be better to require explicit opt-in to the Applicative
> desugaring on a per-do-notation/comprehension basis. Of course, finding
> good syntax is always such a bother...
>
> I'm definitely +1 on the overall idea though, I have a bunch of FRP code
> (where I have to use Applicative) that looks just like Dan Doel's second
> snippet and it's pretty horrid.
>
> Regards,
> Reid Barton
>
> _______________________________________________
> Glasgow-haskell-users mailing list
> Glasgow-haskell-users at haskell.org
> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/glasgow-haskell-users/attachments/20131002/4c220b92/attachment-0001.html>