CODEWITHSUNDEEP
haskell
user17441617
user17441617

Reputation: 21

No instance for Eq

I have a custom data type Point Int Int and I need to compare it to another Point Int Int (with elem function). Could you please tell me how to write Eq instance for this data type? Thanks.

type Result = [String]

data Point = Point Int Int
-- missing Eq instance
data Line = Line Point Point

drawField :: [Point] -> (Int, Int) -> Result
drawField positions (width, height) = let
    check coordinates | elem coordinates positions = '#'
                      | otherwise = '.'
    in [[check (Point rows cols) | cols <- [0..width-1]] | rows <- [0..height-1]]

Upvotes: 1

Views: 112

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476574

If two Points are equal if all parameters are elementwise equal, then you can let Haskell derive the Eq instance with:

data Point = Point Int Int deriving Eq

If the points are equivalent if a more sophisticated test succeeds, you can define your own instance of Eq with:

instance Eq Point where
    Point xa ya == Point xb yb = …

where … is an expression with Bool as type.

Upvotes: 8

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