On May 9, 2008, at 15:56 , Andrew Coppin wrote:
Bryan O'Sullivan wrote:
Andrew Coppin wrote:
But here's a
question: what is the purpose of the MonadPlus class?
It gives you a way of working with monads as monoids. Consider a
Parsec
example:
metasyntactic = text "foo" `mplus` text "bar" `mplus` text "baz"
You'll get back whichever one matched, in left-to-right-order, or
mzero
(a parse failure) if all of them fail.
...so it's a kind of choice operator? Run all actions until you get
to one that succeeds and return the result from that?
In monadic guise that's how it's usually used, yes (since monoids
like (+) are not generally all that useful as monads; (++) (list
monad) and (.) (Monad ((-) r)) might be, though).
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED]
system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED]
electrical and computer engineering, carnegie mellon university KF8NH
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
