How to use trailing closure in if condition?

Question

Here is the code:

class Person {
}

func lastNameForPerson(person: Person, caseFolding: ((String)->(String))? = nil) -> String {
    if let folder = ca         


        

Answer 1

Put your function call between parentheses:

if "SMITH" == (lastNameForPerson(Person()) {$0.uppercaseString}) {

otherwise the == operator takes precedence and the compiler evaluates it as

if ("SMITH" == lastNameForPerson(Person())) {$0.uppercaseString} {

which is not valid code.

Answer 2

You have to put parentheses around the function call:

if "SMITH" == (lastNameForPerson(Person()) {$0.uppercaseString}) {
    print("It's bob")
}

Or you put them around the == comparison (around the if condition) in a C-style manner:

if ("SMITH" == lastNameForPerson(Person()) {$0.uppercaseString}) {
    print("It's bob")
}

Alternatively, you can move the closure inside the parameter list (though this requires you to explicitly name the parameter):

if "SMITH" == lastNameForPerson(Person(), caseFolding: {$0.uppercaseString}) {
    print("It's bob")
}

The reason this problem arises is that the if statement 'claims' the {} block, i.e. it doesn't belong to the lastNameForPerson call any more. For the compiler, the second code block now looks like a normal block that wasn't properly separated from the previous (if) statement.

---

You should probably consider avoiding a construct like this in general, since it might be hard to read (at first). Instead, you could store the result of the function call in a variable and compare that instead:

let lastName = lastNameForPerson(Person()) {$0.uppercaseString}
if "SMITH" == lastName {
    print("It's bob")
}