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POHow to avoid ugly code resolving this problem in Haskell (LANGUAGE extentions)?
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copied!<p>I'm trying to write a program that simulates several creatures in a world. Basically the word sends a message over a list of creatures, and each creature gives his response, which in turn modifies the world.</p> <p>I simplified what I'm trying to write in the following skeleton:</p> <pre><code>module Structure0 where type Message = String class Creature a where processInput :: a -> Message -> Message class World a where processAction :: a -> b -> Message -> a getCreatures :: a -> [b] ---- USAGE EXAMPLE ---- data Parrot = Parrot Int deriving Show instance Creature Parrot where processInput p s = s data ParrotWorld = ParrotWorld [Parrot] instance World ParrotWorld where processAction w p s = w getCreatures (ParrotWorld ps) = ps </code></pre> <p>In this code, I would that the parameter b in World class definition could assume all the data value that belong to the Creature class, something like:</p> <pre><code>processAction :: (Creature b) => a -> b -> Message -> a </code></pre> <p>Of course this examples aren't actual haskell code, lo let's pass illustrating two solution i found: the first, involving ExistentialQuantification:</p> <pre><code>{-# LANGUAGE ExistentialQuantification #-} module Structure1 where type Message = String class Creature_ a where processInput :: a -> Message -> Message data Creature = forall c. Creature_ c => Creature c instance Creature_ Creature where processInput (Creature c) = processInput c class World a where processAction :: a -> Creature -> Message -> a getCreatures :: a -> [Creature] ---- USAGE EXAMPLE ---- data Parrot = Parrot Int deriving Show instance Creature_ Parrot where processInput u s = s data ParrotWorld = ParrotWorld [Creature] instance World ParrotWorld where processAction w p s = w getCreatures (ParrotWorld ps) = ps </code></pre> <p>and the second, suggested by a kind guy on #haskell, using TypeFamilies:</p> <pre><code>{-# LANGUAGE TypeFamilies, FlexibleContexts #-} module Structure2 where type Message = String class Creature a where processInput :: a -> Message -> Message class (Creature (WorldCreature a)) => World a where type WorldCreature a :: * processAction :: a -> WorldCreature a -> Message -> a getCreatures :: a -> [WorldCreature a] ---- USAGE EXAMPLE ---- data Parrot = Parrot Int deriving Show instance Creature Parrot where processInput p s = s data ParrotWorld = ParrotWorld [Parrot] instance World ParrotWorld where type WorldCreature ParrotWorld = Parrot processAction w p s = w getCreatures (ParrotWorld ps) = ps </code></pre> <p>The main objective of this exercise is writing nice, elegant code. So, the questions:</p> <p>1) Should I express Creature as a Class instead of a Data? (I'm doing this because a Creature is just a thing that implement the processInput function, and the many actual Creature implementations vary a lot; expecially during prototyping, I'd like not to change constantly the way in which a Creature pattern-matches.</p> <p>2) The first solution I provide it's a bit ugly due to the boilerplate of maintaing both Creature and Creature_ versions. It has the benefit, however, that I can write mixed list of type [Creature]; Problem is that I can't pattern match against an object, id est things like:</p> <pre><code>\(Creature (Parrot x)) -> x </code></pre> <p>will fail due to type system. Can I make this all right?</p> <p>3) The second solution has a problem of extendibility: say I would to construct a World with two types of creatures, say Parrot1 and Parrot2: how could I write the code in that case?</p> <p>4) Am I structuring the code from a wrong point of view? Can I get an elegant solution just using plain haskell?</p> <p>Thank you all :)</p> <p>Carlo</p>

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