PureScript中Eq类型类实例实现报错，请求排查解决

Let's walk through the issues in your code and get it compiling smoothly:

1. The Root Cause of the Compilation Error #

The critical mistake here is a typo in your Eq instance's eq function:

• You wrote p1.y1 == p2.y2, but your Point data type only has fields x and y—there's no y1 or y2 defined. This is exactly what's triggering the line 17 compilation error.

2. Bonus: Clean Up Redundant Code #

Beyond the typo, you can simplify the eq function: PureScript lets you return boolean expressions directly, so the if ... then true else false wrapper is unnecessary and adds extra noise.

Corrected Full Code #

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 = p1.x == p2.x && p1.y == p2.y

Key Changes Explained #

• Fixed the typo: Changed p1.y1 == p2.y2 to p1.y == p2.y to match the Point type's actual field names.
• Removed redundant logic: The boolean comparison result can be returned directly, making the code cleaner and more aligned with idiomatic PureScript practices.

内容的提问来源于stack exchange，提问作者Knows Not Much