Hi Tim,

Congrats on your choice of tic-tac-toe. I also believe it's a good toy to get your feet wet with haskell.

1. Your concerns about writing a "custom datatype" are best addressed by reading up on it. If you search for "haskell data type declarations", you'll find good material explaining the difference between "data", "newtype", and "type" declarations. You'll also understand the meaning of the error message and what "data constructor" refers to.

2. Functions defined on lists are here:

https://hackage.haskell.org/package/base/docs/Data-List.html

The update function you're looking for is (!!).

-- Kim-Ee

On Fri, Mar 13, 2015 at 6:02 AM, 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 

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