Desugaring do-notation to Applicative

[Iavor Diatchki]

Hello,

I talked to Simon PJ about this at ICFP, and the use case that I'm
interested in is the one where we infer an `Applicative` constraint instead
of `Monad` for things of the form:

do { x1 <- e1; x2 <- e2; ...; pure (f x1 x2 ...) }

as long as the `xs` do not appear in the `es`.   I am interested in this
because I find it quite natural when writing `Applicative` computations.

I am less keen on the more complex rules where part of the `do` notation is
translated using the `Applicative` operations and part using `Monad` ones,
mostly because explaining the translation is somewhat complex.  Perhaps it
is worth noting that if we just implemented the simple rule, we could still
gain the benefits of using `Applicative` (e.g., efficiency) in a part of a
monadic `do` by simply nesting the `do` blocks.  For example:

do x1 <- e1

   -- The following part is `Applicative`
   (x2,x3) <- do x2 <- e2 x1
                 x3 <- e3
                 pure (x2,x3)

   f x1 x2 x3

Either way, I think that it'd be nice to have some version of this feature,
thanks for implementing it!

-Iavor



On Tue, Oct 1, 2013 at 1:56 PM, Dag Odenhall <dag.odenhall at gmail.com> wrote:

> At least Control.Category has RULES that exploit the category laws for
> optimization. Whether this counts as *GHC* making assumptions, I don't
> know. :-)
>
>
> On Tue, Oct 1, 2013 at 10:39 PM, Richard Eisenberg <eir at cis.upenn.edu>wrote:
>
>> The soundness of this desugaring depends on the Applicatives and Monads
>> following some of the laws. I think that's OK, personally, but this
>> assumption should be made loudly somewhere, and the feature should be
>> opt-in. As far as I know, GHC currently makes no assumptions about lawful
>> class instances, and it might cause very strange failures if this suddenly
>> were to change without opting in.
>>
>> Richard
>>
>> On Oct 1, 2013, at 9:49 AM, Simon Peyton-Jones wrote:
>>
>> > What happens when there are some monad things that precede the
>> applicative bit:
>> >
>> > do { x <- e1
>> >   ; y <- e2[x]
>> >   ; z <- e3[x]
>> >   ; h[y]
>> >   ... }
>> >
>> > does this convert to
>> >
>> > do { x <- e1
>> >   ; (y,z) <- (,) <$> e1 <*> e2
>> >   ; h[y]
>> >   ...
>> >
>> > I assume so, but it would be good to say.
>> >
>> >
>> > Also worth noting that join can be used to eliminate the tuple in
>> arbitrary contexts, not only ones that end in return.  Eg in the above
>> example we can desugar to
>> >
>> >       e1 >>= \x ->
>> >       join (\y z -> do { h[y]; ... })
>> >           e2 e3
>> >
>> > I think.  Again worth documenting if so.
>> >
>> > I don't know whether the tuple-version or join-version would be more
>> optimisation friendly.
>> >
>> > Simon
>> >
>> > | -----Original Message-----
>> > | From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
>> > | bounces at haskell.org] On Behalf Of Simon Marlow
>> > | Sent: 01 October 2013 13:40
>> > | To: glasgow-haskell-users
>> > | Subject: Desugaring do-notation to Applicative
>> > |
>> > | Following a couple of discussions at ICFP I've put together a proposal
>> > | for desugaring do-notation to Applicative:
>> > |
>> > |    http://ghc.haskell.org/trac/ghc/wiki/ApplicativeDo
>> > |
>> > | I plan to implement this following the addition of Applicative as a
>> > | superclass of Monad, which is due to take place shortly after the 7.8
>> > | branch is cut.
>> > |
>> > | Please discuss here, and I'll update the wiki page as necessary.
>> > |
>> > | Cheers,
>> > | Simon
>> > | _______________________________________________
>> > | Glasgow-haskell-users mailing list
>> > | Glasgow-haskell-users at haskell.org
>> > | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>> > _______________________________________________
>> > Glasgow-haskell-users mailing list
>> > Glasgow-haskell-users at haskell.org
>> > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>>
>> _______________________________________________
>> Glasgow-haskell-users mailing list
>> Glasgow-haskell-users at haskell.org
>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>>
>
>
> _______________________________________________
> 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/20131001/5be80607/attachment-0001.html>