On Fri, Jan 16, 2009 at 4:04 PM, Jonathan Cast <jonathancc...@fastmail.fm> wrote:
> On Fri, 2009-01-16 at 14:16 +0100, david48 wrote: >> Part of the problem is that something like a monoid is so general that >> I can't wrap my head around why going so far in the abstraction. >> For example, the writer monad works with a monoid; using the writer >> monad with strings makes sense because the mappend operation for lists >> is (++), now why should I care that I can use the writer monad with >> numbers >> which it will sum ? > > To accumulate a running count, maybe? A fairly common pattern for > counting in imperative languages is > > int i = 0; > while (<get a value>) i+= <count of something in value> > > Using the writer monad, this turns into > > execWriter $ mapM_ (write . countFunction) $ getValues well thank you for the example, if I may ask something: why would I need to write a running count this way instead of, for example, a non monadic fold, which would probably result in clearer and faster code (IMHO) ? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
