data Piece = X | O

means that X and O are constants, having data type 'Piece'.

In other words, you directly use X and O instead of Piece X and Piece O.

Piece is a type, and X and O are constructors (or simply the two possible values).

A good similar example is the Bool type,

    data Bool = False | True

    -- Two possible values, i.e. False and True

For more info, take a look here: http://en.wikibooks.org/wiki/Haskell/Type_declarations

Also, to make life easier, you might want to use:

    data Piece = X | O

      deriving Show -- Make this type showable

With this, you will be able to use 'print' with elements of this datatype.

For a tutorial on setting up emacs, take a look here: https://github.com/serras/emacs-haskell-tutorial/blob/master/tutorial.md

For updating nested lists, you have to create a function that takes two numbers (the positions) and iterates over the whole structure, just updating the required position.

While this may seem like an overkill, it gets optimized by ghc.

I don't have any experience with lenses, but they should be usable here. Understanding them will require a good understanding of the type system.

On 13 March 2015 at 04:32, Timothy Washington <twashing@gmail.com> wrote:

To get started, I'm trying to implement a simple tictactoe game. And I would like to be able to represent a Piece on the board, as either the string "X" or "O". This is what I have so far. 

module Main where

data Piece = X | O

type Row = [Piece]

type Board = [Row]

-- put an X or O in a position

move :: Board -> Piece -> Board

move board piece = board

  

-- check win vertically

-- check win horizontally

-- check win diagonally

main :: IO ()

main = putStrLn "Hello World"

A) Now, I'd like to be able to load code interactively, preferably within emacs. However I don't have access to my types with ghci or ghc-mod (Interactive-Haskell). In either case, this call fails with the below error.

let p = Piece X

<interactive>:20:9-13: Not in scope: data constructor `Piece'

B) And how do I make a custom datatype that's one of two strings (enumeration of either "X" or "O"). Cabal builds and runs the abouve code, so I know it can compile. But I'm confused as to where X or O is defined, and how I would supply it as an input. 

C) Finally, how do we update nested lists in Haskell. I want the move function to take a Board, Piece, and Position, and return a Board. I see some results from Hoogle. Is this where Lenses or Zippers come into play?

Thanks 

Tim Washington 

Interruptsoftware.com 

_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

--

Regards

Sumit Sahrawat

_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners