[Haskell-cafe] using haskell for a project

Eugene Kirpichov ekirpichov at gmail.com
Sun May 3 09:46:29 EDT 2009

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-belka at 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
> --
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Eugene Kirpichov
Web IR developer, market.yandex.ru

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