Desugaring do-notation to Applicative

[Greg Weber]

I think there are 2 use cases:
* explicit ado is best, it communicates the intent of the writer and can
give better error messages
* we want users to write code in a do style and the implementer to make it
applicative if possible

So we probably need to accommodate 2 use cases with 2 extensions, one that
enables `ado` and have another extension that will automatically work with
`do`.
Another advantage of this approach is that `ado` should be simpler to get
started.


On Wed, Oct 2, 2013 at 1:27 PM, Bardur Arantsson <spam at scientician.net>wrote:

> On 2013-10-02 20:13, Reid Barton wrote:
> > On Wed, Oct 2, 2013 at 1:50 PM, Edward Kmett <ekmett at gmail.com> wrote:
> >
> >> That is admittedly a pretty convincing example that we may want to
> provide
> >> either a LANGUAGE pragma or a different syntax to opt in.
> >>
> >
> > I suppose the Applicative desugaring can reliably be disabled by adding a
> > syntactic dependency on previous variables, like
> >
> >     [ (x, y) | x <- [1..3], y <- const [1..10000000] x ]
> >
> > so as far as I'm concerned it's sufficient if the Applicative desugaring
> is
> > opt-in on a per-module basis, without a separate syntax for Applicative
> vs
> > Monad do-notation/comprehensions.
>
> That seems like an easily-overlooked and IMO too-subtle way to saying
> "hey, GHC, don't do the applicative desugaring in this particular place".
>
> > Those who opt in can be expected to
> > understand and deal with this sharing issue if it affects them. (They
> > pretty much have to understand it already, if they are compiling with
> > optimizations.)
> >
>
> I don't think it's a about understanding -- not all READERS of the code
> could necessarily be expected to have the same expertise (or level of
> carefulness) as the writer of the code. This could lead to subtle bugs
> arising during maintenance. Therefore it would seem a good idea to me to
> be explicit about the distiction with "ado" vs. "do" (or similar) -- not
> sure about how the distincation should be made in the comprehensions,
> but I'm sure *something* explicit can be worked out. I mean, is a single
> extra letter really a burden?
>
> Regards,
>
> Bardur
>
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