3 ms·

I like the 'weakly pure' concept in D. A function like pure int frignate(database db, const config cfg); cannot change anything except the database object (

• thinkpad20 10y ago
  I'm not sure how D's purity system works, but that doesn't seem very pure to me. Mutating the database object is a globally visible effect, after all.
  • buzzybee 10y ago
    The guarantee it's making is that it's not going to manipulate program state that is outside the scope of "db". That is a pretty big deal for a systems developer since, as I discovered while working with D, many standard library functions that you wouldn't have given a second thought about happen to do unpure things like set a processor flag. We aren't even talking about your own program. From an abstracted viewpoint, it's not great, since a whole database covers potentially a lot of scope, and you may not want to care about the details of your floating point calculations in hardware, but in a concrete sense this is totally correct!
• mej10 10y ago
  You can achieve this level of control using monad transformers or extensible effects -- this is a standard technique in Haskell. frignate :: (MonadDB m) => Config -> m Int frignate cfg = do db <- getDB ... return 1 And it is composable, so if a function calls a function that uses one of the managed resources then the requirement propagates upward. And you can swap in non-IO based instances for testing, or whatever else you want.