Desugaring do-notation to Applicative

[Dag Odenhall]

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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/glasgow-haskell-users/attachments/20131001/132739d1/attachment.html>