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Remove .self and freely allow type references in expressions

Introduction

Swift's grammar currently requires that type references only appear as part of a constructor call T(x) or member access T.x. To get the metatype object for T, one must refer to the special member T.self. I propose allowing type references to appear freely in expressions and removing the .self member from the language.

Swift-evolution thread: Making .self After Type Optional

Motivation

The constructor-or-member restriction on type references exists to provide grammatical and semantic disambiguation between type references and other expressions:

  • Like C++, Java, and C#, Swift adopts angle bracket syntax for generic parameters, T<U>. In expression contexts, this introduces a grammar ambiguity with operators < and >. The expression T<U>(x) could be parsed either as a constructor call, (T<U>)(x), or as a chain of comparisons, (T<U)>(x). Rather than follow in C++'s footsteps of relying on name lookup to resolve the parse, which introduces ugly interdependencies between parsing and semantic analysis, Swift borrowed the heuristic grammatical approach taken by Java and C# to disambiguate these cases. When parsing an expression, when we see an identifier is followed by an opening angle bracket T<, we attempt to parse a potential generic parameter list using the type grammar, and if we reach a matching closing bracket >, we look at the following token. If the token after > is (, ., or one of a few other disambiguating tokens, we accept the parse as a generic parameter list; otherwise, we backtrack and reparse the initial < as an infix operator.

    Though not formally perfect by any means, this heuristic approach works well because a < b > c is unlikely to begin with (and impossible in standard Swift, because Bool is not Comparable in the the standard library), and (a < b, c > (d)) is unlikely as a standalone tuple of expressions due to the already-low precedence of >. Swift adds leading-dot .member expressions over Java and C#, so (a < b, c > .d) is a possibly semantically valid expression, but in practice this has never been reported as a problem.

  • Swift reuses several expression forms as type sugar for common types: T? for Optional<T>, [T] for Array<T>, and [T: U] for Dictionary<T, U>. Since bare type references are not currently allowed in expressions, this avoids conflicts between the type reference and expression forms; [T].self can only be a type reference to Array<T>, since a single-element array containing the type object for T has to be spelled [T.self].

Though precedented by other languages (arguably including Objective-C, since one can only indirectly message classes via [Class method] and cannot reference a Class object directly), this constructor-or-member restriction hasn't been a great fit for Swift, since Swift places a stronger emphasis on type objects. Unlike Java or C#, Swift type objects are first-class, strongly-typed "metatypes", and type objects are readily available without having to go through obscure reflection APIs. Type objects can also be used dynamically in polymorphic contexts via class methods and static protocol requirements, as in Objective-C. Though similar to Objective-C's [T self], Swift's T.self syntax is frequently criticized as obscure, and uglifies code that works heavily with type objects. We should improve our heuristics for dealing with these ambiguities so that .self becomes unnecessary.

Proposed solution

I propose making the following changes:

  • Augment the lookahead rule for parsing generic parameter lists in expressions, so that T<U> type references can be reliably parsed at arbitrary positions within expressions.
  • Treat the disambiguation of T?, [T], and [T: U] as a contextual typing problem, favoring the type reference analysis if possible, but allowing type context to choose the array or dictionary literal interpretation.
  • Remove the now nearly useless magic .self member from the language.

Detailed design

Disambiguating T<U>

To disambiguate T<U> in more expression positions, we can expand the set of lookahead tokens we accept after a potential generic parameter list. Let's enumerate the expression productions in which T<U> is likely to appear:

  • As a top-level statement or variable binding, followed by a newline, semicolon, or enclosing }:

    _ = T<U>; let t = T<U> /* \n */

let makeType: () -> Any.Type = { T<U> }

  • In a ternary, followed by ::

    condition ? T<U> : V<W>

    or, less likely (if some creative developer in future Swift conformed metatypes to BooleanProtocol), as the condition followed by ?:

    T<U> ? x : y

  • On the LHS of a cast, followed by is or as:

    T<U> as Superclass.Type
T<U> is Protocol.Type
T<U> as? Protocol.Type

  • As an element of a container literal, followed by ,, :, or ]:

    [T<U>, V<W>]
[T<U>: V<W>]

  • As a binding in a capture list, also followed by ]:

    {[t = T<U>] in t }

  • As an element of a tuple or argument list, followed by , or ):

    doStuff(withType: T<U>, andOtherType: V<W>)

  • As the root of a constructor call or member access, followed by (, ., or [:

    T<U>(x: 1, y: 2)
T<U>.staticMethod()
T<U>[x] // maybe we'll have `static subscript` one day?

  • As the operand of an operator, followed by a postfix or infix operator token:

    ++T<U>
T<U>++
T<U> + x
T<U>+x

    Operators are interesting because they also potentially require special token-splitting behavior in the lexer to interpret >+ as two tokens rather than a single operator name.

Potential ambiguities arise when > is followed by a token that can also begin a new subexpression. This isn't a problem for many of the tokens enumerated above; , ; : ? } ] ) is and as all unambiguously terminate or continue the current expression and can be safely added to the set of disambiguating tokens. Let's consider the potentially ambiguous cases:

  • Newlines: As the last production in an expression, T<U> may be followed by another statement or declaration on a new line. This is formally ambiguous with an expression involving < and > operators broken across lines:

    a < b >
c

    However, as we noted before, a < b > c is not a semantically correct expression in standard Swift. Furthermore, Swift does not have a comma operator, so a < b, c > d is not a valid expression outside of a tuple, function call, or array literal. For these reasons, I propose that it's safe to say that a potential generic parameter list whose closing angle bracket is followed by a token on a new line is parsed as a generic parameter list.

    // Parses as generic param list `let foo = (a<b>); c`
let foo = a<b>
c
// Parses as operator expr `let foo = (a<b)>c`
let foo = a<b>c

  • Opening brackets: Though we already include ( among the set of disambiguating tokens, we do not include [. We do not currently support static subscript members, so this wouldn't be immediately useful. If we think that's something we may introduce in the future, we could consider adding [ to the set of disambiguating tokens. The expression production (a < b, c > [d]) is theoretically possible, if a function takes multiple unlabeled Bool arguments. This strikes me as slightly more likely than (a < b, c > .d) or (a < b, c > (d)), since array literals are more common than .constants, and array literals don't have the precedence defense that obviates the need to write (a < b, c > (d)).

  • Operators: Swift requires balanced whitespace around infix operators, which can disambiguate between a generic type as the left operand of a binary operator and a prefix operator as the right operand of >:

    a<b> + c  // (a<b>) + c
a<b > +c  // (a<b) > (+c)

    I'm going to go out on a limb and say that's good enough. We could conceive of heroics to decide when to split operator tokens in the a<b>+c case, but in the standard library, the only operators that apply to type objects are != and ==, and while there are some developers who favor "Yoda conditionals" with the constant on the left, the variable == T<U> style with the constant on the right is more common.

In summary, I propose we keep the existing disambiguation rule for generic parameter lists, but expand the list of disambiguating tokens to include . , ; : ? } ] ( ) is and as, spaced binary operators, and any token on a new line. This should let us parse T<U> in expression context reliably enough to eliminate the need for .self as a grammatical disambiguator.

Disambiguating type sugar syntax

The semantic problem of disambiguating type sugar from literal expressions can be considered a contextual typing problem and handled during type checking. If x?, [x], or [x: y] appear in a metatype type context, we can attempt the type reference interpretation. If x? is applied to an optional value x, or [x] appears in ArrayLiteralConvertible context, or [x: y] appears in DictionaryLiteralConvertible context, then we attempt the expression interpretation:

func useType(_ type: Any.Type) {}
func useArray(_ array: [Any.Type]) {}
func useDictionary(_ dict: [Any.Type: Any.Type]) {}

useType(Int?)          // Passes Optional<Int>
useType([Int])         // Passes Array<Int>
useType([Int: String]) // Passes Dictionary<Int, String>

useArray([Int])         // Passes an array, containing Int
useDictionary([Int: String]) // Passes a dictionary, mapping Int => String

The type reference interpretation should still only be valid when the sugar syntax is applied to concrete type references, not metatype variables:

let int = Int
useType([int]) // Error, can't form type reference, and
               // array literal doesn't type-check

If type context is not available, we can favor a type reference if one can be formed, and fall back to the container literal if that's not possible:

let x = [Int] // binds x to the type object Array<Int>
let int = Int
let y = [int] // can't form a type reference, so
              // binds x to an array containing the type object Int

Impact on existing code

If these heuristics are well-chosen, existing code should not be noticeably affected by these changes, other than being liberated from the burdensome .selfs on type references. This is something we should verify experimentally by compiling existing codebases with a compiler that implements these proposed language changes.

Alternatives considered

Change syntax to eliminate the ambiguities

There are various fundamental things we could change in Swift's syntax to eliminate the ambiguities from the language entirely, including:

  • using different brackets for generic type parameters, e.g. Array(Int) or Array[Int];
  • making the UppercaseTypes, lowercaseValues convention a syntactic requirement, as is done in ML and Haskell.

These are directions we rejected early on in the development of Swift, since we felt that maintaining familiarity with C-family languages was worth burning some implementation complexity.

Rationale

On [Date], the core team decided to (TBD) this proposal. When the core team makes a decision regarding this proposal, their rationale for the decision will be written here.