force ( promise -- value )

Vocabulary

promises

Inputs

promise

a promise object

Outputs

value

a factor object

Word description

Calls the quotation associated with the promise if it has not been called before, and returns the value. If the promise has been forced previously, returns the value from the previous call.

Definition

USING: accessors combinators continuations kernel ;

IN: promises

: force ( promise -- value )
    dup status>> {
        { +error+ [ value>> throw ] }
        { +value+ [ value>> ] }
        {
            +unforced+
            [
                dup [
                    quot>> ( -- value ) call-effect >>value
                    +value+ >>status value>>
                ] [ >>value +error+ >>status value>> throw ]
                recover
            ]
        }
    } case ;