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Apple Swift compiler complains about partial application while creating extension method on Array

As an exercise in learning Swift, I'm trying to write minimum and maximum functions for arrays, as shown below:

extension Array {

  func comparest<C: Comparable>(comparator: T -> C, _ op: (C, C) -> Bool) -> T? {
    var min = self.first
      for elem in self {
          if op(comparator(elem), comparator(min!)) {
            min = elem
          }
      }
      return min
  }                     

  func minimum<C: Comparable>(var _ comparator: (T -> C)? = nil) -> T? {
    if comparator == nil {
          comparator = { elem -> C in elem as C }
      }
      return self.comparest(comparator!, <) 
  }

}

I missed the maximum function because it is the same, at least, but with a different operator passed to comparest. I tested the function comparestand it works well, for example.

var array = [4, 2, 3]
var min = array.comparest({ $0 }, <)!

The value of min is 2.

My problem is that the compiler does not like my function minimum. He shows an angry red mistake on the line self.comparest(comparator!, <)and says: "Partial application of the universal method is not allowed." I have written enough Haskell to find out what a partial application is, but for me it does not look like a partial application. All arguments are specified.

?

+3
2

, parens:

var min = array.comparest({ $0 }, (<))!

:

[1,2,3].reduce(0,(+)) // -> 6
+1

, !

, . , , , , Apple.

:

return comparest(comparator!, { $0 < $1 })
+2

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