9. Functions¶

Syntax

FunctionDeclaration ::=
    FunctionQualifierList fn Name GenericParameterList? ( FunctionParameterList? ) ReturnType? WhereClause? (FunctionBody | ;)

FunctionQualifierList ::=
    const? async? unsafe? AbiSpecification?

FunctionParameterList ::=
    (FunctionParameter (, FunctionParameter)* ,?)
  | (SelfParameter (, FunctionParameter)* ,?)

FunctionParameter ::=
    OuterAttributeOrDoc* (FunctionParameterPattern | FunctionParameterVariadicPart | TypeSpecification)

FunctionParameterPattern ::=
    PatternWithoutAlternation (TypeAscription | (: FunctionParameterVariadicPart))

FunctionParameterVariadicPart ::=
    ...

ReturnType ::=
    -> TypeSpecification

FunctionBody ::=
    BlockExpression

SelfParameter ::=
    OuterAttributeOrDoc* (ShorthandSelf | TypedSelf)

ShorthandSelf ::=
    (& LifetimeIndication?)? mut? self

TypedSelf ::=
    mut? self TypeAscription

Legality Rules

9:1 A function is a value of a function type that models a behavior.

9:2 A function declares a unique function item type for itself.

9:3 A function qualifier is a construct that determines the role of a function.

9:4 A function shall not be subject to both keyword async and keyword const.

9:5 A function parameter is a construct that yields a set of bindings that bind matched input values to names at the site of a call expression or a method call expression.

9:6 A self parameter is a function parameter expressed by keyword self.

9:7 A function shall not specify a self parameter unless it is an associated function.

9:8 A function shall not specify a FunctionParameterVariadicPart unless it is an external function.

9:9 The pattern of a function parameter shall be an irrefutable pattern.

9:10 A return type is the type of the result a function returns.

9:11 A function body is the block expression of a function.

9:12 A function shall have a function body unless it is an associated trait function or an external function.

9:13 A function body denotes a control flow boundary.

9:14 A function body of an async function denotes an async control flow boundary.

9:15 A function signature is a unique identification of a function that encompasses of its function qualifiers, name, generic parameters, function parameters, return type, and where clause.

9:16 A constant function is a function subject to keyword const.

9:17 The function body of a constant function shall be a constant expression.

9:18 A constant function shall be callable from a constant context.

9:19 An async function is a function subject to keyword async. An async function of the form

async fn async_fn(param: &param_type) -> return_type {
    /* tail expression */
}

9:20 is equivalent to function

fn async_fn<'a>(param: &'a param_type) -> impl Future<Output = return_type> + 'a {
    async move {
        /* tail expression */
    }
}

9:21 An unsafe function is a function subject to keyword unsafe.

9:22 The invocation of an unsafe function shall require unsafe context.

9:23 A main function is a function that acts as an entry point into a program. A main function is subject to the following restrictions:

• 9:24 It lacks function qualifiers async and unsafe,

• 9:25 Its ABI is Rust,

• 9:26 Its name is the word main,

• 9:27 It lacks generic parameters,

• 9:28 It lacks function parameters,

• 9:29 It lacks a return type,

• 9:30 It lacks a where clause,

• 9:31 It has a function body.

Examples

fn eucledian_distance(left: &Point, right: &Point) -> f64 {
    let x_delta_squared: f64 = (right.x - left.x).powi(2);
    let y_delta_squared: f64 = (right.y - left.y).powi(2);

    (x_delta_squared + y_delta_squared).sqrt()
}

fn main() {}