Functions

Everything about a fn: how one is declared, how its arguments are passed, the one closure form, and the parameter modes a signature can ask for.

What belongs here#

What does not belong here: the members of a type, which are in types/. Every page in this folder is a reference page: an example first, then the syntax, then numbered rules, then what the construct is not.

Pages#

• Declaring a function - fn declares a function with a mandatory parameter type on every parameter; the last expression of the body is the result, and a public function or a trait method must always spell out its return type.
• Arguments and labels - An argument is passed positionally or by label, positional arguments always come first, and a label matches a parameter by name rather than by position.
• Default values - A parameter default is an expression that runs at every call which omits the argument, in the scope of the declaration, without self and without the other parameters.
• Variadic parameters - A parameter written ...name collects every remaining positional argument into a List, and a collection is only unpacked into it when the call spreads it with the same three dots.
• Closures - A brace in expression position is always a closure with inferred or written parameters, var ones too; it captures a const binding as a copy and a var binding as itself, which only a closure handed to a parameter that just calls it may do.
• Trailing closures - When the last parameter of a call is a function, the closure argument can follow the call as a brace instead of sitting inside the parentheses.
• Parameter modes - A parameter is an ordinary value unless it says otherwise; var hands over a path to mutate, lazy defers evaluation once, a self-named closure resolves names against a receiver, and Expression also hands over the typed tree.
• Quoted expressions - A parameter or binding typed Expression gets the ordinary value plus the typed tree of what was written, its source text and the values it captured, which is what assert and a query provider read instead of running the code twice.