Question

我在purescript

中编写了这段代码

module TypeClasses where
  import Prelude
  import Data.Array
  import Data.Number.Format(toString) 

  data Point = Point{x:: Number, y:: Number}
  instance showPoint :: Show Point where 
    show (Point {x, y}) = (toString x) <> ", " <> (toString y)    
  instance eqPoint :: Eq Point where
    eq p1 p2 = if (p1.x == p2.x && p1.y1 == p2.y2) then true else false

但我收到错误

Compiling TypeClasses
Error found:
in module TypeClasses
at src/TypeClasses.purs line 17, column 20 - line 17, column 22

  Could not match type

    { x :: t0
    | t1
    }

  with type

    Point


while checking that type Point
  is at least as general as type { x :: t0
                                 | t1
                                 }
while checking that expression p1
  has type { x :: t0
           | t1
           }
while checking type of property accessor p1.x
in value declaration eqPoint

where t0 is an unknown type
      t1 is an unknown type

See https://github.com/purescript/documentation/blob/master/errors/TypesDoNotUnify.md for more information,
or to contribute content related to this error.

Answer 1

您的代码有几个问题：

您必须解构 Point参数才能访问＆＃34;包裹＆＃34;记录您的实际坐标，如下所示：eq (Point p1) (Point p2)（这将解决您的类型错误）
.y1和.y2不存在，我想你的意思是.y
并提示：if something then true else false可以缩短为something

因此，您最终会Eq Point的{{1}}实施。

instance eqPoint :: Eq Point where
  eq (Point p1) (Point p2) = p1.x == p2.x && p1.y == p2.y

purescript为Eq类创建实现

1 个答案: