To the original author: I must notice that this is not the most convincing use of purity. I personally would prefer to have a character's spell list be mutable because this corresponds better to the nature of the problem (in the problem domain it is nonsense to have two versions of a character and hope that one version will drop out of the scope). However, Haskell forces you to be pure, and that pays off a lot, despite the existence of problems that are not modeled intuitively this way.
In case you absolutely need mutable state, use the ST monad or IORef's, but **you have to first become skilled with pure computations, because that's the only way to not make a mess of your treatment of the impure ones**. And if anyone tries to seduce you with unsafePerformIO, resist! 2009/5/3 Belka <lambda-be...@yandex.ru>: > > Welcome to Haskell! =) > >> But now I don't know how to dynamically add new spells (new spells can be >> created in my gameplay). Since I can't assign a new value to the `spells' >> variable (Data.Map.insert returns a new map), I just don't know where to >> go. > > Do distinguish *pure function* concept and *action with side effects* > concept. > The default in FP (Funct. Progr.) paradigm usualy is *pure function*, if > *action with side effects* is avoidable. Pure functions deal with immutable > memory "objects". So in your case, whenever you add a new spell, you get a > new character (since your chacter's "essence" is a spell list). That way > after calculating *addNewSpell* you will have 2 versions of character: a new > one (with spells list bigger), an the original one. Since old one usualy > drop out of the scope (while program evaluation progresses) it gets *garbage > collected* (note: garbage collection showed up in FP already in 1960s). > > addNewSpell :: Character -> Spell -> Character > addNewSpell char_old_version spell_to_add = ... > > main = let char_new_version = addNewSpell char_old_version new_spell in > putStrLn $ show char_new_version > where char_old_version = Charcter {...} > new_spell = Spell { .... } > > Have luck, with the brain rewriting! =) > Belka > -- > View this message in context: > http://www.nabble.com/using-haskell-for-a-project-tp23348425p23352598.html > Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Eugene Kirpichov Web IR developer, market.yandex.ru _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe