Re: [Haskell-beginners] Dipping Toes Into Haskell

On Mar 23, 2015, at 10:38 PM, Timothy Washington wrote:

[snip]

So ultimately I want a function signature that lets me pass in a lens position.

-- 1. these 2 don't compile together

data Position = Position Int Int deriving Show

move :: Board -> Piece -> Position -> Board move board piece position = set (position) piece board

-- 2. and using the (:t ...) type definition abouve, none of these work

move :: Board -> Piece -> ((Field2 s t a b, Functor f) => (a -> f b) -> s -> f t) -> Board -- move :: Board -> Piece -> ((a -> f b) -> s -> f t) -> Board -- move :: Board -> Piece -> (a -> f b) -> Board -- move :: Board -> Piece -> (s -> f t) -> Board

move board piece position = set (position) piece board

-- 3. so the below code compiles, but doesn't do me much good... I need Position to be a lens, such that I can use A) set (position) piece board , instead of B) set (_2._1) 42 board

module Main where

import Control.Lens

data Piece = X | O | E deriving Show type Row = [Piece] type Board = [Row] data Position = Int Int deriving Show

move :: Board -> Piece -> Position -> Board move board piece position = board

You seem set on lenses. So if you write that move function like this (in case anyone’s wondering, I happen to know this isn’t a homework question): data Piece = X | O | E deriving Show move :: a -> b1 -> ASetter a b a1 b1 -> b move board position piece = board & position .~ piece This’ll work. But how are you going to get the position? If you’re given an integer based coordinate, as you seem to want from your definitions of Position, then you’re going to have to do something ugly. Why not just go with an array and be done with it? Something like this (with your original definition of Piece): import Data.Array data Piece' = X | O | E deriving Show type Position' = (Int,Int) type Board' = Array Position’ Piece' board' :: Board' board' = array ((1,1),(3,3)) [((i,j), E) | i <- [1,2,3], j <- [1,2,3]] move' :: Board' -> Piece' -> Position' -> Board' move' board piece pos = board // [(pos, piece)] If you want a slightly less ugly of looking at the board import qualified Data.List.Split as S pp board = mapM_ print $ S.chunksOf 3 $ elems board will display the board something like: [E,E,E] [E,E,E] [E,E,E] I hope I didn’t say too much. Cheers, Bob

main :: IO () main = putStrLn "Hello World"

Cheers mate :)

Tim Washington Interruptsoftware.com

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