foreignreturn.go

package golog

// simulate an algebraic datatype representing the return value
// of foreign predicates

import "github.com/mndrix/golog/term"

// ForeignReturn represents the return type of ForeignPredicate functions.
// Values of ForeignReturn indicate certain success or failure conditions
// to the Golog machine.  If you're writing a foreign predicate, see
// functions named Foreign* for possible return values.
type ForeignReturn interface {
	IsaForeignReturn() // useless method to restrict implementations
}

// ForeignTrue indicates a foreign predicate that succeeds deterministically
func ForeignTrue() ForeignReturn {
	t := true
	return (*foreignTrue)(&t)
}

type foreignTrue bool

func (*foreignTrue) IsaForeignReturn() {}

// ForeignFail indicates a foreign predicate that failed
func ForeignFail() ForeignReturn {
	f := false
	return (*foreignFail)(&f)
}

type foreignFail bool

func (*foreignFail) IsaForeignReturn() {}

// ForeignUnify indicates a predicate that succeeds or fails depending
// on the results of a unification.  The list of arguments must have
// an even number of elements.  Each pair is unified in turn.  Variables
// bound during unification become part of the Golog machines's bindings.
func ForeignUnify(ts ...term.Term) ForeignReturn {
	if len(ts)%2 != 0 {
		panic("Uneven number of arguments to ForeignUnify")
	}
	return (*foreignUnify)(&ts)
}

type foreignUnify []term.Term

func (*foreignUnify) IsaForeignReturn() {}