Desugaring do-notation to Applicative

[Richard Eisenberg]

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
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