我在使用 PureScript 类型类时遇到问题。我必须先说，我也不是 Haskell 专家，所以如果这些是明显的错误，我深表歉意。
我尝试了几种不同的方法，并且每种方法都碰壁了。我基本上是在尝试为show图中的边定义一个函数。一种方法如下所示：
module Foo where
data Edge n = Edge { from :: n, to :: n }
instance showEdge :: (Show n) => Show (Edge n) where
show e = "Edge from "++(show e.from)++" to "++(show e.to)
e = Edge { from: 1, to: 2 }
main = show e
这给了我错误：
$ psc src/Attempt1.purs
Error at src/Attempt1.purs line 6, column 27:
Error in declaration showEdge
Cannot unify Prim.Object with Foo.Edge.
我猜这与它e推断show. 我尝试在此处添加类型注释，但出现语法错误：
module Foo where
data Edge n = Edge { from :: n, to :: n }
instance showEdge :: (Show n) => Show (Edge n) where
show e :: Edge n
show e = "Edge from "++(show e.from)++" to "++(show e.to)
e = Edge { from: 1, to: 2 }
main = show e
我尝试的第二件事是：
module Foo where
type Edge n = { from :: n, to :: n }
instance showEdge :: (Show n) => Show (Edge n) where
show e = "Edge from "++(show e.from)++" to "++(show e.to)
e :: Edge Number
e = { from: 1, to: 2 }
main = show e
这给了我：
$ psc src/Attempt2.purs
Error at src/Attempt2.purs line 5, column 1:
Type synonym instances are disallowed
所以我然后尝试明确列出底层类型：
module Foo where
type Edge n = { from :: n, to :: n }
instance showEdge :: (Show n) => Show { from :: n, to :: n } where
show e = "Edge from "++(show e.from)++" to "++(show e.to)
e :: Edge Number
e = { from: 1, to: 2 }
main = show e
这给了我：
$ psc src/Attempt3.purs
Error at src/Attempt3.purs line 5, column 1:
Error in type (to :: n, from :: n):
Type class instance head is invalid.
我不知道“类型类实例头”是什么，所以我无处可去。
三个尝试都失败了。可能出于完全不同的原因。作为 PureScript 的新手，我只是不知道问题出在哪里。我一直在尝试查看Data.*各种类型的示例并通过示例阅读 PureScript。我一直无法弄清楚这一点。
感谢您的任何帮助。