[Haskell-cafe] Comments from OCaml Hacker Brian Hurt

david48 dav.vire+haskell at gmail.com
Sat Jan 17 04:47:56 EST 2009

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

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