Re: [Haskell-cafe] Comments from OCaml Hacker Brian Hurt

On Thu, 2009-01-15 at 18:41 -0500, Cale Gibbard wrote:

2009/1/15 Andrew Coppin :

...

OK, well then my next question would be "in what say is defining configuration files as a monoid superior to, uh, not defining them as a monoid?" What does it allow you to do that you couldn't otherwise? I'm not seeing any obvious advantage, but you presumably did this for a reason...

I can't speak from the perspective of the Cabal developers, but combining configurations with partial information using a monoid operation is generally a good way to structure things. Basically, this would be analogous to the way that the First monoid (or the Last monoid) works, but across a number of fields. You have an empty or default configuration which specifies nothing that serves as the identity, and then a way of layering choices together, which is the monoid operation.

Exactly. Some fields are the Last monoid (we call it Flag) and some are the list monoid. Whole sets of such settings are monoids point-wise. It is indeed great for combining/overriding setting from defaults, config files and the command line. Duncan