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Clarify Function Type Restriction on Argument Label
Location
https://docs.swift.org/swift-book/documentation/the-swift-programming-language/types#Function-Type
Description
The following is an excerpt from the Function Type subsection of the Types section:
Argument names in functions and methods aren’t part of the corresponding function type. For example:
func someFunction(left: Int, right: Int) {} func anotherFunction(left: Int, right: Int) {} func functionWithDifferentLabels(top: Int, bottom: Int) {}
var f = someFunction // The type of f is (Int, Int) -> Void, not (left: Int, right: Int) -> Void. f = anotherFunction // OK f = functionWithDifferentLabels // OK
func functionWithDifferentArgumentTypes(left: Int, right: String) {} f = functionWithDifferentArgumentTypes // Error
func functionWithDifferentNumberOfArguments(left: Int, right: Int, top: Int) {} f = functionWithDifferentNumberOfArguments // Error
Because argument labels aren’t part of a function’s type, you omit them when writing a function type.
var operation: (lhs: Int, rhs: Int) -> Int // Error var operation: (_ lhs: Int, _ rhs: Int) -> Int // OK var operation: (Int, Int) -> Int // OK
The sentence between the examples is slightly misleading. The code may either provide neither an argument label or parameter name (last line of the second example) or provide the wildcard as the argument label with a parameter name (middle line of the second example).
Correction
Append a second sentence between the two examples to read:
Because argument labels aren’t part of a function’s type, you omit them when writing a function type. This can be done either by providing neither an argument label nor parameter name or by providing a wildcard argument label with a parameter name.
