使用＆＃39; map＆＃39;时输入错误 - 哈斯克尔

Question

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)

我有一个点（x，y）列表，我想改变Y坐标的值。当我尝试编译这个我收到这个错误：

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)

我猜我没有正确使用地图。任何提示解决方案的提示都表示赞赏。 ： - ）

Answer 1

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需要2个参数，但你传递的是3个。

尝试在flipOnesY调用周围加上括号，如下所示：

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

这也向您显示您对flipsOneY的参数顺序不是最佳的。