In swift, can a function be a type?

Question

While researching XCTAssert methods by pressing command+click, it looks like they underlying method is a function that has a type (a generic type referred to as T, that conforms to the Equatable protocol). Am I saying this correctly, and if so how do functions comform to protocols? Are functions types?

public func XCTAssertEqual<T : Equatable>(_ expression1: @autoclosure () throws -> ArraySlice<T>, _ expression2: @autoclosure () throws -> ArraySlice<T>, _ message: @autoclosure () -> String = default, file: StaticString = #file, line: UInt = #line)

This line is the most confusing which I'm trying to explain above:

func XCTAssertEqual<T : Equatable>`

Answer 1

Every function has a specific function type, made up of the parameter types and the return type of the function.

typealias FooType = (Int, String)->String

func foo(i: Int, s: String)->String {
    return s + "_\(i)"
}

func bar(foo0: FooType)->String {
    return foo0(100, "alpha")
}

print(bar(foo))         // alpha_100

let f:FooType = { i, s in
    return s + "_\(i)"
}

print(f(200, "beta"))   // beta_200

an appendix, especially for Brandon :-)

let farr:[()->Int] = [{return 1}, {return 2}, {return 3}]
for f in farr {
    print(f())
/*
 1
 2
 3
*/
}

appendix 2, for Brandon

func f1()->Int {
    return 1
}
func f2()->Int {
    return 2
}

let farr2:[()->Int] = [f1, f2]
for f in farr2 {
    print(f())
/*
 1
 2
 */
}

app 3 :-)

let farr2 = [f1, f2]
for f in farr2 {
    print(f())
/*
 1
 2
 */
}

from apple docs

Every function in Swift has a type, consisting of the function’s parameter types and return type. You can use this type like any other type in Swift, which makes it easy to pass functions as parameters to other functions, and to return functions from functions. Functions can also be written within other functions to encapsulate useful functionality within a nested function scope.