[Haskell-cafe] Applicative do?

Edward Kmett ekmett at gmail.com
Fri Oct 9 13:36:46 EDT 2009

Good trick. I just added 'ado' to my little scheme monad library. ;)

-Edward Kmett

On Fri, Oct 9, 2009 at 1:06 PM, Philippa Cowderoy <flippa at flippac.org>wrote:

> I do a lot of work with parsers, and want to do more using Applicatives.
> That said, I'm finding it a little tedious being forced to use pointless
> style for a task that's well-suited to having a few names around. The idea
> of an applicative do notation's been kicked around on #haskell a few times
> (I can't find any trace of it on the mailing list, but I confess to not
> having searched too hard), so I thought I'd propose it here.
> The basic idea is to turn this:
> do a <- f
>  g
>  b <- h
>  pure $ foo a b
> into this:
> (\a b -> pure $ foo a b) <*> (f <*> g *> h)
> Aside from changing >>= and >> into <*> and *>, the most significant
> difference from monadic do is that all the generated lambda abstractions go
> in front of the final "return" statement which is then fmapped across the
> rest of the code. Bindings are thus only in scope in the "return" statement.
> I believe sugared let statements can be handled similarly so long as they
> respect the binding discipline.
> This leads us to the bikeshed topic: what's the concrete syntax? The
> obvious way is to replace do with a new keyword - for example, ado for
> "applicative do". There's a nice alternative though: we can check whether a
> do statement meets the binding rules for an applicative block and treat it
> as one if so, or a monadic one if not. While not all Monads are
> Applicatives, code can readily be changed back using the WrappedMonad
> newtype - whereas existing code needn't turn on the appropriate extension in
> the first place.
> Thoughts, comments?
> --
> flippa at flippac.org
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