I'm trying to find some practical applications of a comonad and I thought I'd try to see if I could represent the classical Wumpus world as a comonad.
I'd like to use this code to allow the Wumpus to move left and right through the world and clean up dirty tiles and avoid pits. It seems that the only comonad function that's useful is extract (to get the current tile) and that moving around and cleaning tiles would not use be able to make use of extend or duplicate.
I'm not sure comonads are a good fit but I've seen a talk (Dominic Orchard: A Notation for Comonads) where comonads were used to model a cursor in a two-dimensional matrix.
If a comonad is a good way of representing the Wumpus world, could you please show where my code is wrong? If it's wrong, could you please suggest a simple application of comonads?
module Wumpus where
-- Incomplete model of a world inhabited by a Wumpus who likes a nice
-- tidy world but does not like falling in pits.
import Control.Comonad
-- The Wumpus world is made up of tiles that can be in one of three
-- states.
data Tile = Clean | Dirty | Pit
deriving (Show, Eq)
-- The Wumpus world is a one dimensional array partitioned into three
-- values: the tiles to the left of the Wumpus, the tile occupied by
-- the Wumpus, and the tiles to the right of the Wumpus.
data World a = World [a] a [a]
deriving (Show, Eq)
-- Applies a function to every tile in the world
instance Functor World where
fmap f (World as b cs) = World (fmap f as) (f b) (fmap f cs)
-- The Wumpus world is a Comonad
instance Comonad World where
-- get the part of the world the Wumpus currently occupies
extract (World _ b _) = b
-- not sure what this means in the Wumpus world. This type checks
-- but does not make sense to me.
extend f w@(World as b cs) = World (map world as) (f w) (map world cs)
where world v = f (World [] v [])
-- returns a world in which the Wumpus has either 1) moved one tile to
-- the left or 2) stayed in the same place if the Wumpus could not move
-- to the left.
moveLeft :: World a -> World a
moveLeft w@(World [] _ _) = w
moveLeft (World as b cs) = World (init as) (last as) (b:cs)
-- returns a world in which the Wumpus has either 1) moved one tile to
-- the right or 2) stayed in the same place if the Wumpus could not move
-- to the right.
moveRight :: World a -> World a
moveRight w@(World _ _ []) = w
moveRight (World as b cs) = World (as ++ [b]) (head cs) (tail cs)
initWorld = World [Dirty, Clean, Dirty] Dirty [Clean, Dirty, Pit]
-- cleans the current tile
cleanTile :: Tile -> Tile
cleanTile Dirty = Clean
cleanTile t = t
Thanks!
