[Haskell-cafe] Comments from OCaml Hacker Brian Hurt

Jonathan Cast jonathanccast at fastmail.fm
Fri Jan 16 10:04:50 EST 2009

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


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