swift - 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 = caseFolding {
        return folder("Smith")
    }
    return "Smith"
}

print(lastNameForPerson(Person())) // Prints "Smith"
print(lastNameForPerson(Person()) {$0.uppercaseString}) // Prints "SMITH"

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

Was expecting to get "It's bob". But instead got the error:

Consecutive statements must be separated by a new line

Answer 1

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")
}