Optional parameters

A function parameter can be made optional using the ? modifier after the parameter name; this means that the caller is not required to pass an argument to it.

When an optional parameter is omitted, the variable won't exist in the function body; as a result, accessing it can fail, causing an error. To prevent this from happening, you need to use a fallback expression to provide a default value.

Please note that all optional parameters must appear after all required parameters, and before any variadic parameter; breaking this order will cause a compiler error.

# Generates a map for a pet with a name and species (defaults to "dog")
[$gen-pet: name; species?] {
    (::
        name = <name>;
        species = <species ? "dog">; # Fallback to "dog" if species is undefined
    )
}

Default values

Optional parameters can also define a default expression directly in the signature:

[$gen-pet: name; species ? "dog"] {
    (::
        name = <name>;
        species = <species>;
    )
}

For ordinary parameters, this default is eager: if the caller omits species, Ranty evaluates "dog" immediately when the call is bound.

Lazy optional parameters

Prefix an optional parameter with @lazy to defer the caller's argument until the parameter is first accessed:

[$show-subtitle: @lazy subtitle?] {
    <subtitle ? "(none)">
}

If subtitle is omitted here, it is still absent and the fallback remains necessary. @lazy changes when a provided value is evaluated; it does not make missing optional parameters automatically exist.

Lazy optional parameters can also have lazy defaults:

[$title-line: @lazy title ? [pick-title]] {
    Title: <title>
}

If title is omitted, [pick-title] is captured and deferred until <title> is first accessed. If title is never read, the default expression never runs.

See also Lazy parameters and Fallbacks.