Desugaring do-notation to Applicative

[Reid Barton]

On Wed, Oct 2, 2013 at 12:01 PM, Dag Odenhall <dag.odenhall at gmail.com>wrote:

> What about MonadComprehensions, by the way? The way I see it, it's an
> even better fit for Applicative because the return is implicit.
>
Yes, or ordinary list comprehensions for that matter.

But there is a danger in desugaring to Applicative: it may introduce too
much sharing. Currently a program like "main = print $ length [ (x, y) | x
<- [1..3], y <- [1..10000000] ]" (or the equivalent in do-notation) runs in
constant space with either -O0 or -O -fno-full-laziness. If you desugar it
to a form like "main = print $ length $ (,) <$> [1..3] <*> [1..10000000]",
then no optimization flags will save you from a space leak.

It might be better to require explicit opt-in to the Applicative desugaring
on a per-do-notation/comprehension basis. Of course, finding good syntax is
always such a bother...

I'm definitely +1 on the overall idea though, I have a bunch of FRP code
(where I have to use Applicative) that looks just like Dan Doel's second
snippet and it's pretty horrid.

Regards,
Reid Barton
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