PureScript与类型化

Question

我和PureScript打字员有问题。我不得不说，坦白说，我也不是Haskell专家，所以如果这些都是明显的错误，我很抱歉。

我试过几种不同的方法，每种方法都碰到了一堵墙。我基本上是想为图中的一个边定义一个show函数。一种方法如下：

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

这给了我一个错误：

$ psc src/Attempt1.purs
Error at src/Attempt1.purs line 6, column 27:
Error in declaration showEdge
Cannot unify Prim.Object with Foo.Edge.

我猜想这与在e的定义中推断show的类型有关。我尝试在这里添加一个类型注释，但是我得到了一个语法错误：

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

我试过的第二件事是：

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

于是，我尝试显式地列出底层类型：

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，所以我没有地方可走了。

三次尝试都失败了。可能是因为完全不同的原因。作为PureScript的新手，我只是不知道问题出在哪里。我一直试图继续研究各种Data.*类型的示例，并逐个阅读PureScript。我还没搞清楚。

谢谢你的帮助。

Answer 1

实际上，在第一次尝试时，您几乎已经做到了，这里的问题是Edge是一个包含一个对象的字段的数据构造函数，而Haskell中相同的语法是定义函数来访问数据中的几个字段。

Haskell没有像PureScript那样像第一类对象那样有对象/记录，所以您所需要做的就是从Edge中打开对象

show (Edge e) = "Edge from " ++ show e.from ++ " to " ++ show e.to