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
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 https://www.haskell.org/hoogle/?hoogle=update. Is this where Lenses https://www.haskell.org/hoogle/?hoogle=lens or Zippers https://www.haskell.org/hoogle/?hoogle=zipper come into play?
Thanks
Tim Washington Interruptsoftware.com http://interruptsoftware.com
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