Question

我想计算多边形的面积，但是我实现的代码不起作用。有人能帮助我找到更好的解决方案吗？

这是我的代码：

ar :: (Float,Float,Float)-> Float
ar (v1,v2,v3) = let a=distBetween v1 v2
        b=distBetween v2 v3
        c=distBetween v3 v1
        s=0.5*(a+b+c)
        in sqrt (s*(s-a)*(s-b)*(s-c))

distBetween :: (Float,Float)->(Float,Float)->Float
distBetween (x1,y1) (x2,y2) = sqrt ((x1-x2)^2+(y1-y2^2))

这是我的错误：

Type error in application
*** Expression     : distBetween v1 v2
*** Term           : v2
*** Type           : Float
*** Does not match : (Float,Float)

Answer 1

distBetween适用于两对Float，但v1，v2和v3都只是单Float个。如果你有

-- Just creates an alias for `(Float, Float)`, since otherwise it'd be a lot of typing
type Point = (Float, Float)

然后你可以把它写成

distBetween :: Point -> Point -> Float
distBetween (x1, y1) (x2, y2) = sqrt $ (x1-x2)^2 + (y1-y2)^2
-- You also had a typo in your distance function here    ^

ar :: (Point, Point, Point) -> Float
ar (v1, v2, v3) =
    let a = distBetween v1 v2
        b = distBetween v2 v3
        c = distBetween v3 v1
        s = 0.5 * (a + b + c)
    in sqrt $ s * (s - a) * (s - b) * (s - c)

然后它会编译得很好。既然你已告诉编译器v1，v2和v3都只是Float，那么它就不能与(Float, Float)匹配根据{{1}}的参数需要。

多边形区域 - Haskell

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