Re: [Haskell-beginners] Dipping Toes Into Haskell

Hey man... On Mon, Mar 23, 2015 at 4:57 AM, Bob Hutchison wrote:

Hi Tim,

Straying from clojure? :-)

Lol, love it!! I'm a free spirit. Can't hold me down :)

On Mar 22, 2015, at 12:33 PM, Timothy Washington wrote:

So I've finally had a chance to revisit my tictactoe game.

import Control.Lens

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

-- put an X or O in a position *move :: Board -> Piece -> Position -> Board* move board piece position = board

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

Now, if I want to make a move, I'll want to set a *piece* on a *position*, on a *board*. Let's assume the board has the following shape.

λ> let r1 = (E,E,E) λ> let r2 = (E,E,E) λ> let r3 = (E,E,E) λ> let board = (r1,r2,r3) ((E,E,E),(E,E,E),(E,E,E))

To return an updated board, I dug around and *i)* couldn't find a core Haskell equivalent to Clojure's update-in http://clojuredocs.org/clojure.core/update-in. *ii)* Zippers only deal with trees of left / right nodes. *iii)* So that left me with Control.Lens https://github.com/ekmett/lens/wiki/Examples. Now I can easily inspect and update the board like so.

Or you could use an two dimensional array of Positions instead (Data.Array).

Hmm, probably. But the function's type signature below, seem to be the first order of business, no?

λ> board^*._**2._1* E λ> set (*_2._1*) 42 board ((E,E,E),(42,E,E),(E,E,E))

I can then parameterize _2 or _1 like so.

λ> let a = _2 λ> board ^. a (E,E,E)

You can probably find the type of _1, _2, _3 in the repl, and use that in your code below.

The types (:t ...) of both of these are: - a :: (Field2 s t a b, Functor f) => (a -> f b) -> s -> f t - _2 :: (Field2 s t a b, Functor f) => (a -> f b) -> s -> f t

But I can't figure out how to include that parameter's type into a *Position* type definition. I've tried these unsuccessfully.

*data Position = Int Int deriving Show -- original type definition* *data Position = Simple Lens (Int a) (Int a) deriving Show -- failing try 1*

These are very suspicious. Did you intend to define constructors ‘Int’ and ‘Simple’? Maybe you meant something like:

data Position = Position Int Int deriving Show

In which case you’ll have a type called ‘Position’ and a constructor with the same name.

Cheers, Bob

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 main :: IO () main = putStrLn "Hello World" Cheers mate :) Tim Washington Interruptsoftware.com http://interruptsoftware.com/