Swift: String can't be nil, so why can stringByAddingPercentEncodingWithAllowedCharacters be nil?

Question

I'm relatively new to Swift and I've been wrapping my head around optional variables. Here's the issue I've run into:

func escape(s: String) -> String
{
    if s == nil
    {
        return ""
    }

    return s.stringByAddingPercentEncodingWithAllowedCharacters(.URLHostAllowedCharacterSet())
}

The compiler complains that:

Value of type 'String' can never be nil, comparison isn't allowed

This is in reference to the if s == nil line. If I remove the if statement line and just have the return value line, I get the error:

Value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'

I see in the docs that stringByAddingPercentEncodingWithAllowedCharacters returns a String? parameter. I don't understand why this isn't forced to be a String? Since a String can't be nil, I can't pass nil to this function, so how would it return nil?

This is a duplicate of http://stackoverflow.com/q/33558933/292145 — Klaas, Mar 24 '16 at 12:24

score 2 · Answer 1 · answered Nov 14 '15 at 13:03

You defined the parameter s as non optional String. A non optional type can never be nil, that's why the compiler complains.
Just remove the checking for nil.

You defined the return type as a non optional String.
Either you have to unwrap the result value to conform to the type

func escape(s: String) -> String
{
  return s.stringByAddingPercentEncodingWithAllowedCharacters(.URLHostAllowedCharacterSet())!
}

or change the return type to an optional

func escape(s: String) -> String?
{
  return s.stringByAddingPercentEncodingWithAllowedCharacters(.URLHostAllowedCharacterSet())
}

score 1 · Answer 2 · answered Nov 14 '15 at 12:57

Not a minute after posting I read deeper into the docs. stringByAddingPercentEncodingWithAllowedCharacters CAN return a nil value, even given a non-nil value. From https://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSString_Class/#//apple_ref/occ/instm/NSString/stringByAddingPercentEncodingWithAllowedCharacters

Return Value Returns the encoded string, or nil if the transformation is not possible.

Discussion UTF-8 encoding is used to determine the correct percent encoded characters. Entire URL strings cannot be percent-encoded.

Swift: String can't be nil, so why can stringByAddingPercentEncodingWithAllowedCharacters be nil?

2 Answers2

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