Creating data type in Haskell

Question

I would like to create a new data type that either takes in Start or A tuple of values. This is what I have:

type Coord = (Int, Int)
data Direction = N | E | S | W
                 deriving (Eq, Show, Read)


type Move = (Coord, Direction)
type BoardSpec = [(Coord, Bool)]

type GameTreeNode = (GameTree2, BoardSpec, Move)
data GameTree2 = Start | GameTreeNode deriving(Show)


blahblah :: GameTree2
blahblah = blahb

blahb :: GameTreeNode
blahb = (Start, testbo, ((1, 1), N))

This however, gives me the error on blahblah

Couldn't match type `(GameTree2, Move)' with `GameTree2'
Expected type: GameTree2
  Actual type: GameTreeNode

Bartek Banachewicz · Accepted Answer · 2014-12-01 15:50:25Z

2

~~data GameTree2 = Start | GameTreeNode deriving(Show)~~

expands to:

data GameTree2 = Start | (GameTree2, BoardSpec, Move) deriving(Show)

You can't make a tuple a valid value of your type.

You need to wrap it in a constructor:

data GameTree2 = Start | Node GameTreeNode deriving(Show)

where Node is whatever name you choose for it.

edited Dec 1, 2014 at 15:50

answered Dec 1, 2014 at 14:12

Bartek Banachewicz

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2 Comments

Daniel Wagner Over a year ago

The proposed solution is nice, but the explanation of what went wrong is not correct. data GameTree2 = Start | GameTreeNode does not expand at all; it simply defines a new name GameTreeNode at the term level that is completely distinct from the GameTreeNode that exists at the type level.

Bartek Banachewicz Over a year ago

@DanielWagner oh my, what an oversight on my part.

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