[Haskell-cafe] Applicative do?
ekmett at gmail.com
Fri Oct 9 14:04:08 EDT 2009
I have idiom brackets in that toy library already, but the ado syntax is
fairly useful if you want to refer to several intermediate results by name.
To work with idiom brackets you need to manually write a big lambda yourself
and them apply it. If you have a lambda that takes several arguments --
which isn't all that uncommon in a parser! -- the names you are binding and
their position in the input can get rather far apart, even using idiom
sugar. Philippa's ado sugar lets you amortize that big binding statement
over several lines and keeps the names closer to the binding.
Not sure that it warrants a syntax extension in the Haskell case, but it is
pretty convenient. =)
On Fri, Oct 9, 2009 at 1:52 PM, Daniel Peebles <pumpkingod at gmail.com> wrote:
> I'd prefer "idiom brackets" over something do-ish for Applicatives.
> Conor McBride's SHE already supports them, if you're willing to use a
> custom preprocessor.
> On Fri, Oct 9, 2009 at 1:45 PM, Joe Fredette <jfredett at gmail.com> wrote:
> > The only issue I would have with such a notation is not being able to
> > visually tell the difference between a monadic function (say, without a
> > explicit type sig, which is how I write parsers), and an applicative one.
> > I'd prefer something like
> > foo = app
> > blah blah
> > If only for some visual distinction, I think it also resolves the "do
> > knowing about types" issue.
> > Plus, this is a good case for some kind of custom-do syntax facility. So
> > could make do syntax for everything. :)
> > /Joe
> > On Oct 9, 2009, at 1:11 PM, Robert Atkey wrote:
> >> On Fri, 2009-10-09 at 18:06 +0100, Philippa Cowderoy wrote:
> >>> This leads us to the bikeshed topic: what's the concrete syntax?
> >> I implemented a simple Camlp4 syntax extension for Ocaml to do this. I
> >> chose the syntax:
> >> applicatively
> >> let x = foo
> >> let y = bar
> >> in <pure stuff>
> >> I quite like the word "applicatively".
> >> Your overloading suggestion sounds to me like it would require the
> >> desugaring process to know something about types, but I'm not sure.
> >> Bob
> >> --
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