[Haskell-beginners] Dipping Toes Into Haskell

[Timothy Washington]

Good explanation. Thank-you.


Tim Washington
Interruptsoftware.com <http://interruptsoftware.com>


On Thu, Mar 12, 2015 at 7:50 PM, Sumit Sahrawat, Maths & Computing, IIT
(BHU) <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> 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 at 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 <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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>>
>>
>
>
> --
> Regards
>
> Sumit Sahrawat
>
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