Function Type Aliases

A function type describes a callable signature — its parameter types and its return type — without naming a particular function. It is written with the func keyword followed by the parameters and an optional -> return type:

func(a: int32, b: int32) -> int32;   // takes two int32, returns int32
func(x: int32, y: int32);            // takes two int32, returns nothing
func() -> bool;                      // takes nothing, returns a bool

These types tend to be long and appear repeatedly wherever callbacks are passed around, which makes them the prime candidate for a type alias:

type Comparator = func(a: int32, b: int32) -> int32;

Using a Named Signature

Once aliased, the signature reads as a single concept. Use it for a parameter that receives a callback:

type Comparator = func(a: int32, b: int32) -> int32;

func Sort(items: Slice<int32>, cmp: Comparator) {
    // ... compares elements with cmp(items[i], items[j]) ...
}

Any function whose signature matches the alias can be passed by name — the alias is transparent, so no cast is involved:

func Ascending(a: int32, b: int32) -> int32 {
    return a - b;
}

func Descending(a: int32, b: int32) -> int32 {
    return b - a;
}

Sort(data, Ascending);    // pass the function by name
Sort(data, Descending);   // a different function, same Comparator type

Calling Through the Value

Inside Sort, the parameter cmp is called like any function:

let order = cmp(x, y);    // invokes whichever function was passed

Parameter Names Are Documentation

Only the parameter types and the return type determine whether a function matches a function type; the parameter names in the type are there for readability. These two aliases describe the same type:

type Comparator = func(a: int32, b: int32) -> int32;
type Cmp        = func(int32, int32) -> int32;       // names omitted

Signatures Without a Return

A function type that omits -> matches functions that return nothing — the usual shape for event handlers and callbacks invoked for their side effects:

type Handler = func(x: int32, y: int32);

func OnClick(x: int32, y: int32) {
    Print(x, y);
}

func Register(h: Handler) { /* … */ }

Register(OnClick);

See Also

• Type Aliases — the alias overview and transparency
• Using Type Aliases — shortening signatures and documenting intent
• Function Declaration — how the functions you pass are written