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Functions

This section is a work-in-progress experiment about making the book executable.

A function consists of a block (that’s the body of the function), along with a name, a set of parameters, and an output type. Other than a name, all these are optional.

Syntax
Function: qualifiers=FunctionQualifiers fn name=IDENTIFIER generic_params=GenericParams?
        ( parameters=FunctionParameters? )
        ( -> return_type=Type )? where_clauses=WhereClauses?
        body=FunctionBody
    => Function { qualifiers, name, generic_params: generic_params.unwrap_or_default(), parameters: parameters.unwrap_or_default(), return_type, where_clauses: where_clauses.unwrap_or_default(), body, }
FunctionBody:
    | BlockExpression => FunctionBody::Block(BlockExpression)
    | ; => FunctionBody::Missing

Function qualifiers FunctionQualifiers fn name IDENTIFIER generic_params GenericParams ( parameters FunctionParameters ) -> return_type Type where_clauses WhereClauses body FunctionBody 
pub struct Function {
    pub qualifiers: FunctionQualifiers,
    /// The name of the function.
    pub name: Identifier,
    /// Generic parameters, for polymorphic functions.
    pub generic_params: GenericParams,
    /// The arguments that the function takes.
    pub parameters: Vec<FunctionParam>,
    /// The type returned by the function.
    pub return_type: Option<Type>,
    /// Additional predicates that must hold when calling the function.
    pub where_clauses: WhereClauses,
    /// The function body.
    pub body: FunctionBody,
}

pub enum FunctionBody {
    /// The body of a function is a block.
    Block(BlockExpression),
    /// Method declarations and `extern` definitions need not have a body.
    Missing,
}
Syntax
FunctionQualifiers: is_const=const? is_async=async? safety=ItemSafety? extern_abi=ExternAbi?
    => FunctionQualifiers { is_const: is_const.is_some(), is_async: is_async.is_some(), safety, extern_abi, }
ItemSafety:
    | safe => ItemSafety::Safe
    | unsafe => ItemSafety::Unsafe
ExternAbi: extern abi=Abi?
    => ExternAbi { abi }

FunctionQualifiers is_const const is_async async safety ItemSafety extern_abi ExternAbi
ItemSafety safe unsafe 
pub struct FunctionQualifiers {
    pub is_const: bool,
    pub is_async: bool,
    pub safety: Option<ItemSafety>,
    pub extern_abi: Option<ExternAbi>,
}

pub enum ItemSafety {
    Safe,
    Unsafe,
}

pub struct ExternAbi {
    pub abi: Option<String>,
}
Syntax
FunctionParameters: first_arg=FunctionParam remaining_args=( , FunctionParam )* ,?
    => [first_arg].into_iter().chain(remaining_args).collect()
FunctionParam: attrs=OuterAttribute* kind=FunctionParamKind
    => FunctionParam { attrs, kind }
FunctionParamKind:
    | & lifetime=Lifetime? mutability=Mutability self => FunctionParamKind::RefSelfShorthand { lifetime, mutability }
    | mutability=Mutability self ty=( : Type )? => FunctionParamKind::SelfParam { mutability, ty }
    | pattern=( PatternNoTopAlt : )? ty=FunctionParamType => FunctionParamKind::Regular { pattern, ty }
FunctionParamType:
    | Type => FunctionParamType::Type(Type)
    | ... => FunctionParamType::Variadic


pub struct FunctionParam {
    pub attrs: Vec<OuterAttribute>,
    pub kind: FunctionParamKind,
}

pub enum FunctionParamKind {
    Regular {
        pattern: Option<Pattern>,
        ty: FunctionParamType,
    },
    SelfParam {
        mutability: Mutability,
        ty: Option<Type>,
    },
    RefSelfShorthand {
        lifetime: Option<Lifetime>,
        mutability: Mutability,
    },
}

pub enum FunctionParamType {
    Type(Type),
    Variadic,
}