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1682-field-init-shorthand.md

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Summary

When initializing a data structure (struct, enum, union) with named fields, allow writing fieldname as a shorthand for fieldname: fieldname. This allows a compact syntax for initialization, with less duplication.

Example usage:

struct SomeStruct { field1: ComplexType, field2: AnotherType }

impl SomeStruct {
    fn new() -> Self {
        let field1 = {
            // Various initialization code
        };
        let field2 = {
            // More initialization code
        };
        SomeStruct { field1, field2 }
    }
}

Motivation

When writing initialization code for a data structure, the names of the structure fields often become the most straightforward names to use for their initial values as well. At the end of such an initialization function, then, the initializer will contain many patterns of repeated field names as field values: field: field, field2: field2, field3: field3.

Such repetition of the field names makes it less ergonomic to separately declare and initialize individual fields, and makes it tempting to instead embed complex code directly in the initializer to avoid repetition.

Rust already allows similar syntax for destructuring in pattern matches: a pattern match can use SomeStruct { field1, field2 } => ... to match field1 and field2 into values with the same names. This RFC introduces symmetrical syntax for initializers.

A family of related structures will often use the same field name for a semantically-similar value. Combining this new syntax with the existing pattern-matching syntax allows simple movement of data between fields with a pattern match: Struct1 { field1, .. } => Struct2 { field1 }.

The proposed syntax also improves structure initializers in closures, such as might appear in a chain of iterator adapters: |field1, field2| SomeStruct { field1, field2 }.

This RFC takes inspiration from the Haskell NamedFieldPuns extension, and from ES6 shorthand property names.

Detailed design

Grammar

In the initializer for a struct with named fields, a union with named fields, or an enum variant with named fields, accept an identifier field as a shorthand for field: field.

With reference to the grammar in parser-lalr.y, this proposal would expand the field_init rule to the following:

field_init
: ident
| ident ':' expr
;

Interpretation

The shorthand initializer field always behaves in every possible way like the longhand initializer field: field. This RFC introduces no new behavior or semantics, only a purely syntactic shorthand. The rest of this section only provides further examples to explicitly clarify that this new syntax remains entirely orthogonal to other initializer behavior and semantics.

Examples

If the struct SomeStruct has fields field1 and field2, the initializer SomeStruct { field1, field2 } behaves in every way like the initializer SomeStruct { field1: field1, field2: field2 }.

An initializer may contain any combination of shorthand and full field initializers:

let a = SomeStruct { field1, field2: expression, field3 };
let b = SomeStruct { field1: field1, field2: expression, field3: field3 };
assert_eq!(a, b);

An initializer may use shorthand field initializers together with update syntax:

let a = SomeStruct { field1, .. someStructInstance };
let b = SomeStruct { field1: field1, .. someStructInstance };
assert_eq!(a, b);

Compilation errors

This shorthand initializer syntax does not introduce any new compiler errors that cannot also occur with the longhand initializer syntax field: field. Existing compiler errors that can occur with the longhand initializer syntax field: field also apply to the shorthand initializer syntax field:

  • As with the longhand initializer field: field, if the structure has no field with the specified name field, the shorthand initializer field results in a compiler error for attempting to initialize a non-existent field.

  • As with the longhand initializer field: field, repeating a field name within the same initializer results in a compiler error (E0062); this occurs with any combination of shorthand initializers or full field: expression initializers.

  • As with the longhand initializer field: field, if the name field does not resolve, the shorthand initializer field results in a compiler error for an unresolved name (E0425).

  • As with the longhand initializer field: field, if the name field resolves to a value with type incompatible with the field field in the structure, the shorthand initializer field results in a compiler error for mismatched types (E0308).

Drawbacks

This new syntax could significantly improve readability given clear and local field-punning variables, but could also be abused to decrease readability if used with more distant variables.

As with many syntactic changes, a macro could implement this instead. See the Alternatives section for discussion of this.

The shorthand initializer syntax looks similar to positional initialization of a structure without field names; reinforcing this, the initializer will commonly list the fields in the same order that the struct declares them. However, the shorthand initializer syntax differs from the positional initializer syntax (such as for a tuple struct) in that the positional syntax uses parentheses instead of braces: SomeStruct(x, y) is unambiguously a positional initializer, while SomeStruct { x, y } is unambiguously a shorthand initializer for the named fields x and y.

Alternatives

Wildcards

In addition to this syntax, initializers could support omitting the field names entirely, with syntax like SomeStruct { .. }, which would implicitly initialize omitted fields from identically named variables. However, that would introduce far too much magic into initializers, and the context-dependence seems likely to result in less readable, less obvious code.

Macros

A macro wrapped around the initializer could implement this syntax, without changing the language; for instance, pun! { SomeStruct { field1, field2 } } could expand to SomeStruct { field1: field1, field2: field2 }. However, this change exists to make structure construction shorter and more expressive; having to use a macro would negate some of the benefit of doing so, particularly in places where brevity improves readability, such as in a closure in the middle of a larger expression. There is also precedent for language-level support. Pattern matching already allows using field names as the destination for the field values via destructuring. This change adds a symmetrical mechanism for construction which uses existing names as sources.

Sigils

To minimize confusing shorthand expressions with the construction of tuple-like structs, we might elect to prefix expanded field names with sigils.

For example, if the sigil were :, the existing syntax S { x: x } would be expressed as S { :x }. This is used in MoonScript.

This particular choice of sigil may be confusing, due to the already-overloaded use of : for fields and type ascription. Additionally, in languages such as Ruby and Elixir, :x denotes a symbol or atom, which may be confusing for newcomers.

Other sigils could be used instead, but even then we are then increasing the amount of new syntax being introduced. This both increases language complexity and reduces the gained compactness, worsening the cost/benefit ratio of adding a shorthand. Any use of a sigil also breaks the symmetry between binding pattern matching and the proposed shorthand.

Keyword-prefixed

Similarly to sigils, we could use a keyword like Nix uses inherit. Some forms we could decide upon (using use as the keyword of choice here, but it could be something else), it could look like the following.

  • S { use x, y, z: 10}
  • S { use (x, y), z: 10 }
  • S { use {x, y}, z: 10 }
  • S { use x, use y, z: 10}

This has the same drawbacks as sigils except that it won't be confused for symbols in other languages or adding more sigils. It also has the benefit of being something that can be searched for in documentation.