使用'地圖'時輸入錯誤 - Haskell

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) 

我有一個點（x，y）的列表，我想要更改Y座標的值。當我嘗試編譯這個我收到此錯誤：

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) 

我猜我沒有正確使用地圖。任何提示解決方案的讚賞。 :-)

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個參數，但你傳遞它三。

嘗試把括號圍繞flipOnesY調用，就像這樣：

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

這也說明你，你的論點爲了flipsOneY不是最優的。