CODEWITHSUNDEEP
haskelldictionarytypes
Rewbert
Rewbert

Reputation: 347

Type error when using 'map' - Haskell

type Point = (Double, Double)

flipsOneY :: Point -> (Int, Int) -> Point
flipsOneY point (i, j) = ((fst point), ((fromIntegral j) - (snd point)))

changeY :: [Point] -> (Int, Int) -> [Point]
changeY xs (i, j) = map flipsOneY xs (i, j)

I have a list of points (x, y) where i want to change the value of the Y coordinate. When i try to compile this i recieve this error:

Expr.hs:149:21:
Couldn't match expected type `(Int, Int) -> Point'
            with actual type `[(Int, Int) -> Point]'
The function `map' is applied to three arguments,
but its type `(Point -> (Int, Int) -> Point)
              -> [Point] -> [(Int, Int) -> Point]'
has only two
In the expression: map flipsOneY xs (i, j)
In an equation for `changeY':
    changeY xs (i, j) = map flipsOneY xs (i, j)

I am guessing i am not using map correctly. Any hint towards a solution is appreciated. :-)

Upvotes: 0

Views: 108

Answers (1)

Don Stewart
Don Stewart

Reputation: 137997

type Point = (Double, Double)

flipsOneY :: Point -> (Int, Int) -> Point
flipsOneY point (i, j) = ((fst point), ((fromIntegral j) - (snd point)))

changeY :: [Point] -> (Int, Int) -> [Point]
changeY xs (i, j) = map flipsOneY xs (i, j)

map takes 2 arguments but you are passing it three.

Try putting parenthesis around the flipOnesY call, like so:

changeY :: [Point] -> (Int, Int) -> [Point]
changeY xs (i, j) = map (\ys -> flipsOneY ys (i, j)) xs

This also shows you that your argument order to flipsOneY is not optimal.

Upvotes: 3

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