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 <s...@scientician.net>wrote: > On 2013-10-02 20:13, Reid Barton wrote: > > On Wed, Oct 2, 2013 at 1:50 PM, Edward Kmett <ekm...@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 > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >
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