Question

我在使用PureScript类型类时遇到了麻烦。我必须在前面说，我不是Haskell专家，所以如果这些是明显的错误我会道歉。

我尝试了几种不同的方法并为每种方法打了一堵墙。我基本上试图为图中的边定义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.

我猜这与它在e的定义中推断show的类型这一事实有关。我尝试在这里添加一个类型注释，但是我收到了一个语法错误：

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

因此我尝试明确列出基础类型：

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

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。

所有三次尝试都失败了。可能出于完全不同的原因。作为PureScript的新手，我只是不知道问题是什么。我一直在尝试跟踪各种Data.*类型的示例并按示例阅读PureScript。我无法弄清楚这一点。

感谢您的帮助。

Answer 1

实际上，你第一次尝试就差不多了，你在这里遇到的问题是Edge是一个数据构造函数，其中一个字段包含一个对象，而Haskell中的相同语法是定义用于访问数据中多个字段的函数。

Haskell没有像PureScript那样将对象/记录作为第一类对象，因此您需要做的就是从Edge打开对象：

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

PureScript和类型类

1 个答案: