On Fri, Jan 16, 2009 at 4:04 PM, Jonathan Cast
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) ?