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Alternative abbreviated lambdas


This paper provides an alternative abbreviated lambda expression syntax, as well as an alternative syntax for transparent functions, compared to [@P0573R2].

Motivation and scope

[@P0573R2] introduces a notion of transparent functions, which aim to behave as close as possible to as if their body was directly inserted into the caller (except that their arguments are only computed a single time).

As described in the mentioned proposal, current function declarations and lambda-expressions have multiple problems with transparent functions:

  • They take arguments and return by value by default (auto), while we usually want to accept forwarding references and retain ref-qualifier for the return type (decltype(auto)). Copies should be explicit
  • They are not noexcept-correct, i.e. they are not automatically noexcept if all the expressions within their body are non-throwing
  • They aren't SFINAE-friendly by default

There are additional problems not mentioned in [@P0573R2]:

  • Function declarations are not constexpr-correct, i.e. they aren't automatically constexpr if they satisfy the requirements for a constexpr function
  • Another problem is discussed in the following subsection


Lambda expressions are meant to reduce boilerplate, but today's lambdas, especially short ones, are cumbersome to use. Compare to other programming languages:

  • A lambda returning 42:
    • In Haskell: \() -> 42 (ignoring the nuances)
    • In Java: () -> 42
    • In Kotlin: { 42 }
    • In Swift: { 42 }
    • In C#: () => 42
    • In Rust: || 42
    • In C++: [] { return 42; }
  • A lambda multiplying its argument by 2:
    • In Haskell: \x -> x * 2
    • In Java: x -> x * 2
    • In Kotlin: { x -> x * 2 }
    • In Swift: { x in x * 2 }
    • In C#: x => x * 2
    • In Rust: |x| x * 2
    • In C++: [](auto&& x) { return x * 2; }
  • A lambda that adds its arguments:
    • In Haskell: \x y -> x + y
    • In Java: (x, y) -> x + y
    • In Kotlin: { x, y -> x + y }
    • In Swift: { x, y in x + y }
    • In C#: (x, y) => x + y
    • In Rust: |x, y| x + y
    • In C++: [](auto&& x, auto&& y) { return x + y; }

Other programming languages:

  • Do not require an explicit capture clause, assuming capture by reference
  • Do not require explicit parameter types, inferring them from context or assuming most general types
  • Do not require an explicit return, assuming it
  • Some languages have special short forms for zero and single-parameter lambdas

[@P0927R2] discusses how their "implicit lambdas" could be replaced with a lambda-based approach, but the syntax for a zero-parameter lambda expression would need to be as terse as possible.

Proposed solution

The primary proposed syntax is |param1, param2, param3| expr, as in Rust.

Such a lambda:

  • Assumes capture by reference [&], except when the lambda is non-local, then assumes no capture []
  • Assumes auto&& declarators for all the parameters (attributes are allowed on the parameters)
  • Is equivalent to a normal lambda with a single return statement, except when the return type is void, then it is equivalent to a normal lambda with an expression-statement
  • Avoids copies by deducing the return type using decltype((expr)). Users will have to make copies explicitly where appropriate, that could be done using auto operator proposed by [@P0849R2]
  • Is SFINAE-friendly
  • Is noexcept-correct: marked as noexcept unless its expression is potentially-throwing

Capture customization is possible using |param1, param2, param3| [captures] expr syntax.

Optional extension: abbreviated lambdas with multiple statements

The syntax in this case is:

|params| [optional-captures] { statement… optional-expr }

Such a lambda:

  • Has all the traits of a single-expression abbreviated lambda, except that…
  • Is not SFINAE-friendly
  • Implicitly returns the tailing (semicolon-less) expression, unless there is none such
  • Deduces the return type using the first return statement or, if there is none such, the trailing expression

Optional extension: transparent function declarations

The syntax in this case is:

auto f(auto&& x, auto&& y) transparent { statement… optional-expr }

Such a function:

  • Is SFINAE-friendly iff the body only consists of the expr
  • Is noexcept-correct
  • Is constexpr-correct
  • Implicitly returns the trailing (semicolon-less) expression, unless there is none such
  • Avoids copies by deducing the return type using decltype((expr-in-first-return)). Users will have to make copies explicitly where appropriate, that could be done using auto operator proposed by [@P0849R2]


Syntax choice

For the purposes of integration with SFINAE-friendliness and noexcept-correctness of [@P0573R2], we will only discuss single-expression lambdas.

[@P0927R2] and some other applications require that the lambda syntax is as brief as possible.

Any abbreviated lambda expression syntax must have a list of parameters (which may consist of zero or one parameter) and an expression, which is its body. It will therefore have a general form of:

… param1, param2, param3 … expr …

Because param1 would otherwise create an ambiguity (consider usage of lambda expression as a higher-order function argument), some separator is required before param1. For clarity when reading, some separator should be required before expr. A separator after expr is not required; expr is then defined to be an assignment-expression. The choice then boils down to choosing the appropriate "framing" of parameters. Several choices have been reviewed:

  • (x, y) f(x) is ambiguous with a C-style cast (single-parameter case) or with a comma-expression (multiple-parameter case)
  • [x, y] f(x) is ambiguous with normal lambda expressions
  • {x, y} f(x) is ambiguous with initializer-lists
  • <x, y> f(x) visually conflicts with <…> usually meaning templates in C++
  • |x, y| f(x) is potentially ambiguous with | and || operators, but not actually, because those are invalid where a lambda-expression can start
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