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 <fli...@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? > > -- > fli...@flippac.org > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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