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POHaskell polymorphic functions with records and class types
 

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    <p>this post is the following of <a href="https://stackoverflow.com/questions/7430520/oo-like-interface-implementation-in-haskell">this one.</a></p> <p>I'm realizing a simple battle system as toy project, the typical system you can find in games like Final Fantasy et simila. I've solved the notorious "Namespace Pollution" problem with a class type + custom instances. For example:</p> <pre><code>type HitPoints = Integer type ManaPoints = Integer data Status = Sleep | Poison | .. --Omitted data Element = Fire | ... --Omitted class Targetable a where name :: a -&gt; String level :: a -&gt; Int hp :: a -&gt; HitPoints mp :: a -&gt; ManaPoints status :: a -&gt; Maybe [Status] data Monster = Monster{monsterName :: String, monsterLevel :: Int, monsterHp :: HitPoints, monsterMp :: ManaPoints, monsterElemType :: Maybe Element, monsterStatus :: Maybe [Status]} deriving (Eq, Read) instance Targetable Monster where name = monsterName level = monsterLevel hp = monsterHp mp = monsterMp status = monsterStatus data Player = Player{playerName :: String, playerLevel :: Int, playerHp :: HitPoints, playerMp :: ManaPoints, playerStatus :: Maybe [Status]} deriving (Show, Read) instance Targetable Player where name = playerName level = playerLevel hp = playerHp mp = playerMp status = playerStatus </code></pre> <p>Now the problem: I have a spell type, and a spell can deal damage or inflict a status (like Poison, Sleep, Confusion, etc):</p> <pre><code>--Essentially the result of a spell cast data SpellEffect = Damage HitPoints ManaPoints | Inflict [Status] deriving (Show) --Essentially a magic data Spell = Spell{spellName :: String, spellCost :: Integer, spellElem :: Maybe Element, spellEffect :: SpellEffect} deriving (Show) --For example fire = Spell "Fire" 20 (Just Fire) (Damage 100 0) frogSong = Spell "Frog Song" 30 Nothing (Inflict [Frog, Sleep]) </code></pre> <p>As suggested in the linked topic, I've created a generic "cast" function like this:</p> <pre><code>--cast function cast :: (Targetable t) =&gt; Spell -&gt; t -&gt; t cast s t = case spellEffect s of Damage hp mana -&gt; t Inflict statList -&gt; t </code></pre> <p>As you can see the return type is t, here showed just for consistency. I want be able to return a new targetable (i.e. a Monster or a Player) with some field value altered (for example a new Monster with less hp, or with a new status). The problem is that i can't just to the following:</p> <pre><code>--cast function cast :: (Targetable t) =&gt; Spell -&gt; t -&gt; t cast s t = case spellEffect s of Damage hp' mana' -&gt; t {hp = hp', mana = mana'} Inflict statList -&gt; t {status = statList} </code></pre> <p>because hp, mana and status "are not valid record selector". The problem is that I don't know a priori if t will be a monster or a player, and I don't want to specify "monsterHp" or "playerHp", I want to write a pretty generic function. I know that Haskell Records are clumsy and not much extensibile...</p> <p>Any idea?</p> <p>Bye and happy coding,</p> <p>Alfredo </p>
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    1. COWhy do players and monsters need to be different types in the first place? They seem to have a lot in common. What is the difference between them?
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    2. COFor now they are quite the same thing, but as design choice I decided to keep them separeted. I can't exclude they will have different field.. just to name one, a monster can have an element (e.g. piros is a fire monster), a player can't :)
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