Functional Programming: This thread is like a burrito

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > Functional Programming: This thread is like a burrito

loose-fish: Apr 1, 2005

While playing around with generic execution models I ran into the following, a typeclass with a function that returns another instance of that typeclass:

code:

class Processor a where
    process :: Processor b => a -> Int -> b

data P = P Int

instance Processor P where
    process (P x) y = P y
-- can't return P because it's no Processor

-- more precise, but pointless
instance (Processor P) => Processor P where
    process (P x) y = P y

Is there a way to achieve something like this in Haskell? Type families maybe? I'm not up to speed on all extensions...

# ¿ May 20, 2017 12:33

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# ¿ May 14, 2024 04:44

loose-fish: Apr 1, 2005

Thanks for clearing that up! I had a fundamental misconception of type classes, guess the 'forall' should've tipped me off...

I'm trying to find a solution for processors with arbitrary local state, and a function to apply an event to the global state according to the current processor. (I say 'State' but I don't care if it's in the State Monad, it's just what I'm using here).
This is the best I've come up with so far:

code:

import Control.Monad.State (State, put)

type Model = String
type Event = Int

data AppState = AppState { _model :: Model
                         , _processor :: Event -> State AppState ()
                         }

class Processor a where
    process :: a -> Event -> State AppState ()


data P = P Int

instance Processor P where
    process (P 0) x = put $ AppState "foo" (process $ Q x (x * 2))  -- change processor to Q
    process (P _) x = put $ AppState (show x) (process $ P x)


data Q = Q Int Int

instance Processor Q where
    process (Q 0 0) x = put $ AppState "bar" (process $ P x)  -- change processor to P
    process (Q _ _) x = put $ AppState (show x) (process $ Q x (x * 2))

I know the simple way to do it would be something like this:

code:

data Processor = P Int | Q Int Int

process :: Processor  Model -> Event -> (Processor, Model)
process (P p) m = processP p m
process (Q q) m = processQ q m

But this would make it impossible to add new Processors in separate modules...

# ¿ May 21, 2017 11:53

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > Functional Programming: This thread is like a burrito