Set.forall F#遇到问题 [英] Trouble with Set.forall F#

问题描述

我在尝试将一个元素与集合中的所有元素进行比较时遇到麻烦.我想编写一个布尔函数，如果一个元素不是邻居，则返回true；如果一个元素是邻居，则返回false.我们想给图表上色 因此，共享边界的两个国家/地区不会使用相同的颜色.我会用代码解释

I'm having trouble trying to compare an element to all elements of a set. I want to write a boolean function that returns true if an element is not a neighbour and false if an element is a neighbour. We want to colour the chart so that two countries that share a border are not given the same colour. I will explain it with code

type Country = string;;
type Chart = Set<Country*Country>;;
type Colour = Set<Country>;;
type Colouring = Set<Colour>;;

(* This is how you tell that two countries are neighbours.  It requires a chart.*)
let areNeighbours ct1 ct2 chart =
  Set.contains (ct1,ct2) chart || Set.contains (ct2,ct1) chart;;
(* val areNeighbours :
  ct1:'a -> ct2:'a -> chart:Set<'a * 'a> -> bool when 'a : comparison
  *)

我在使用canBeExtBy函数时遇到麻烦.如果这是我的图表和我的列:

I'm having trouble with the canBeExtBy function. If this is my chart and my col:

val myWorld : Chart =
  set
    [("Andorra", "Benin"); ("Andorra", "Canada"); ("Andorra", "Denmark");
     ("Benin", "Canada"); ("Benin", "Denmark"); ("Canada", "Denmark");
     ("Estonia", "Canada"); ("Estonia", "Denmark"); ("Estonia", "Finland");
     ...]

col = set 
    ["Canada"]

然后，如果我调用

canBeExtBy col "Denmark" myWorld;;

这是我的代码，出现错误，列在底部.

Here is my code, I get an error which is listed at the bottom.

(* The colour col can be extended by the country ct when they are no neighbours
according to chart.*)

       val canBeExtBy :
      col:Set<'a> -> ct:'a -> chart:Set<'a * 'a> -> bool when 'a : comparison
    *)

错误:

 Set.forall(fun x -> areNeighbours x ct) col;;
  ----------------------^^^^^^^^^^^^^^^^^^
This expression was expected to have type
    bool    
but here has type
    Set<'a * 'a> -> bool  

推荐答案

听您的类型.

This expression was expected to have type
    bool    
but here has type
    Set<'a * 'a> -> bool

有一个类型为Set<'a * 'b> -> bool的函数，而不是布尔值.这是缺少部分最后一个Set<'a * 'b>类型参数的函数的标志.如果查看您的areNeighbours函数，您会看到它带有三个参数，两个为Country类型，一个为Chart类型，但是在canBeExtBy中，您仅向其传递了两个Country值，而不是Chart.

Instead of a boolean value, there is a function of type Set<'a * 'b> -> bool. That is a sign for a partially applied function that's missing its last argument of type Set<'a * 'b>. If you look at your areNeighbours function, you can see that it takes three arguments, two of type Country, and one of type Chart, but in canBeExtBy, you're only passing it two Country values, but not the Chart.

要使其进行编译，canBeExtBy需要看起来像这样:

To make it compile, canBeExtBy needs to look like this:

let canBeExtBy col ct chart =
    Set.forall(fun x -> areNeighbours x ct chart |> not) col

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