functional dependencies vs type families
- by mhwombat
I'm developing a framework for running experiments with artificial life, and I'm trying to use type families instead of functional dependencies. Type families seems to be the preferred approach among Haskellers, but I've run into a situation where functional dependencies seem like a better fit. Am I missing a trick? Here's the design using type families. (This code compiles OK.)
{-# LANGUAGE TypeFamilies, FlexibleContexts #-}
import Control.Monad.State (StateT)
class Agent a where
agentId :: a -> String
liveALittle :: Universe u => a -> StateT u IO a
-- plus other functions
class Universe u where
type MyAgent u :: *
withAgent :: (MyAgent u -> StateT u IO (MyAgent u)) ->
String -> StateT u IO ()
-- plus other functions
data SimpleUniverse = SimpleUniverse
{
mainDir :: FilePath
-- plus other fields
}
defaultWithAgent :: (MyAgent u -> StateT u IO (MyAgent u)) -> String ->
StateT u IO ()
defaultWithAgent = undefined -- stub
-- plus default implementations for other functions
--
-- In order to use my framework, the user will need to create a typeclass
-- that implements the Agent class...
--
data Bug = Bug String deriving (Show, Eq)
instance Agent Bug where
agentId (Bug s) = s
liveALittle bug = return bug -- stub
--
-- .. and they'll also need to make SimpleUniverse an instance of Universe
-- for their agent type.
--
instance Universe SimpleUniverse where
type MyAgent SimpleUniverse = Bug
withAgent = defaultWithAgent -- boilerplate
-- plus similar boilerplate for other functions
Is there a way to avoid forcing my users to write those last two lines of boilerplate? Compare with the version using fundeps, below, which seems to make things simpler for my users. (The use of UndecideableInstances may be a red flag.) (This code also compiles OK.)
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
UndecidableInstances #-}
import Control.Monad.State (StateT)
class Agent a where
agentId :: a -> String
liveALittle :: Universe u a => a -> StateT u IO a
-- plus other functions
class Universe u a | u -> a where
withAgent :: Agent a => (a -> StateT u IO a) -> String -> StateT u IO ()
-- plus other functions
data SimpleUniverse = SimpleUniverse
{
mainDir :: FilePath
-- plus other fields
}
instance Universe SimpleUniverse a where
withAgent = undefined -- stub
-- plus implementations for other functions
--
-- In order to use my framework, the user will need to create a typeclass
-- that implements the Agent class...
--
data Bug = Bug String deriving (Show, Eq)
instance Agent Bug where
agentId (Bug s) = s
liveALittle bug = return bug -- stub
--
-- And now my users only have to write stuff like...
--
u :: SimpleUniverse
u = SimpleUniverse "mydir"
