Option objects provide a way of dealing with values that may or may not be present. The option module provides two constrcutors for option objects: full(_) and empty.

Accessing the option module

The option module object can be imported using import "option" as option, where option is an identifier of your choice. The object option responds to the following requests.

type Option⟦T⟧ = Collection⟦T⟧ & type {
    value → T
    valueIfEmpty⟦U⟧ (eValue:Block0⟦U⟧) → T | U
    isFull → Boolean
    isEmpty → Boolean
    ifFull⟦U⟧ (fAction:Block1⟦T, U⟧) ifEmpty (eAction:Block0⟦U⟧) → U
    ifEmpty⟦U⟧ (eAction:Block0⟦U⟧) ifFull (fAction:Block1⟦T, U⟧) → U
        // same semantics as ifFull(fAction) ifEmpty (eAction), but lets the 
        // programmer write the actions in the opposite order.
    ifFull (fAction:Block1⟦T, Done⟧) → Done
        // same semantics as ifFull (fAction) ifEmpty {done}
    ifEmpty (eAction:Block0⟦Done⟧) → Done
        // same semantics as ifFull (done) ifEmpty (eAction)
}

ValueError → ExceptionKind

full⟦T⟧(contents:T) → Option⟦T⟧
// creates an object s such that s.value and s.valueIfEmpty{...} answer contents, 
// isFull answers true and isEmpty answers false.
// ifFull(fAction) ifEmpty (_) applies fAction to contents and returns its result.
// The object s also behaves exactly like a collection containing the single
// element contents, so, for example, s.do(action) applies action to contents.

empty⟦T⟧ → Option⟦T⟧ 
// creates an object s such that s.value raises a ValueError, and 
// s.valueIfEmpty { expr } executes expr and returns its value; 
// isFull returns false and isEmpty return true.
// ifFull (_) ifEmpty (eAction) executes eAction and returns its result.
// The object s also behaves exactly like an empty collection,
// so, for example, s.do(_) does nothing.

Usage

Option objects provide a way of dealing with values that may not be present. For example, if specifying a color is optional, a variable c that holds the choice of color might take a value that is an option.Option⟦Color⟧.
When deciding how to paint an object, client code might then write something like

    paintWithColor(c.valueIfEmpty {randomColor})

If c is filled with a color, this has the effect of painting with that color; otherwise, it paints with a randomColor.

Alternatives

An alternative to using an option object is to create an application-specific default value, e.g., defaultColor. This object can then be given application-specific method, so that in most cases there is no need to test the value. Another alternative is to use a singleton object, such as noColor, and an explicit match(_)case(_)… to check whether c is noColoror a real color.