[Haskell-cafe] Smarter do notation

Daniel Peebles pumpkingod at gmail.com
Sun Sep 4 06:45:46 CEST 2011

With parsers, for example, it amounts to you have a context-free vs. a
context-sensitive language. The functions hidden behind a monadic bind are
effectively opaque to any sort of analysis, whereas the static structure of
an applicative can be analyzed as much as you want. Ed Kmett does this in
his trifecta parsing library (I think there's a couple of other libraries
that also do this), but you have to use the applicative interface explicitly
where possible to take advantage of the additional optimizations.

This would also have benefits for other sorts of EDSLs, for the same reason.
An applicative computation might for example be sparked and processed in
parallel, whereas it's a lot harder (impossible) to do that if your
structure isn't determined beforehand.

On Sun, Sep 4, 2011 at 12:24 AM, Ivan Lazar Miljenovic <
ivan.miljenovic at gmail.com> wrote:

> On 4 September 2011 12:34, Daniel Peebles <pumpkingod at gmail.com> wrote:
> > Hi all,
> > For example, if I write in a do block:
> > x <- action1
> > y <- action2
> > z <- action3
> > return (f x y z)
> > that doesn't require any of the context-sensitivty that Monads give you,
> and
> > could be processed a lot more efficiently by a clever Applicative
> instance
> > (a parser, for instance).
> What advantage is there in using Applicative rather than Monad for
> this?  Does it _really_ lead to an efficiency increase?
> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> IvanMiljenovic.wordpress.com
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