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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