Trouble with Set.forall F#

问题

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
  *)

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"]

then my function should return false if I call

canBeExtBy col "Denmark" myWorld;;

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
    *)

Error:

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

回答1:

Listen to your types.

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

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.

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

来源：https://stackoverflow.com/questions/35399564/trouble-with-set-forall-f

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