SE-0243: Codepoint and Character Literals

[taylorswift]

@Karl

For those complaining about optionals and performance/trapping: have you heard of the unsafelyUnwrapped property ? It doesn't check or trap in optimised builds:

Isn’t this one of those things they tell us never to use? aiming a gun at your foot shouldn’t be the price you have to pay for an ASCII literal

@RMJay

Single quoted literals should clearly default to Character, which is the contituent element of a string at the level of human interpretation, and is the most common character-like type. This is a particularly beginner visible part of Swift.

Counterpoint: beginner exercises tend to involve processing fixed-length, structured, machine-readable strings. Think of any old CS homework problem—does it involve UTF8 continuations or grapheme breaking? If you want to teach a beginner how to get the query out of a URL string, you don’t want to throw Unicode at them, you just want them to treat/convert the String as an array of ASCII characters. (ASCII characters are actually the only characters valid in a url). There’s been a lot of lament on the forum about how Swift Strings and its unicode-correct indexing model are getting in the way of Swift education and adoption, and I’ve always considered better ASCII support to be the most appropriate solution to this problem.

Unicode is an advanced topic for advanced users, and Characters are Unicode, a lot more than Unicode.Scalar, ironically. Maybe placing this API on the higher shelf isn’t the worst idea in the world.

@RMJay

Character already has an let a = c.asciiValue //Optional(Int) property which is the correct form to use on an arbitrary character parameter. A trapping variant would only be useful when bound to a literal ie.

'a'.ascii //useful
arbitrary.ascii //not useful

I agree, a trapping property is not appropriate on either Character or Unicode.Scalar. There would be no language precedent,, .first and .last are important, commonly used properties, and we do not make them trap.

@RMJay

It seems to me that Swift is missing a langauge feature that would let us have the best overall outcome. That feature being Literal Bound Functions these would allow us to write the following, which is making use of a hypothetical literal bound ascii property on character-literal.

I don’t know how I feel about abusing function call syntax (yes, 'a'.ascii is a function call, just written without parentheses) for literal coercion. Why not the 'A' as UInt8 syntax? I think as is a natural fit for this role, given its current usage in the language, and the literal | as grammar is well-established in Swift, so we wouldn’t have to teach users a new rule “‘literal bound functions’ can only be called on literal values”.