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package golog

import "github.com/mndrix/golog"

Golog aspires to be an ISO Prolog interpreter. It currently supports a small subset of the standard. Any deviations from the standard are bugs. Typical usage looks something like this:

m := NewMachine().Consult(`


    parent(X) :-
    parent(X) :-
if m.CanProve(`father(john).`) {
    fmt.Printf("john is a father\n")

solutions := m.ProveAll(`parent(X).`)
for _, solution := range solutions {
    fmt.Printf("%s is a parent\n", solution.ByName_("X"))

This sample highlights a few key aspects of using Golog. To start, Golog data structures are immutable. NewMachine() creates an empty Golog machine containing just the standard library. Consult() creates another new machine with some extra code loaded. The original, empty machine is untouched. It's common to build a large Golog machine during init() and then add extra rules to it at runtime. Because Golog machines are immutable, multiple goroutines can access, run and "modify" machines in parallel. This design also opens possibilities for and-parallel and or-parallel execution.

Most methods, like Consult(), can accept Prolog code in several forms. The example above shows Prolog as a string. We could have used any io.Reader instead.

Error handling is one oddity. Golog methods follow Go convention by returning an error value to indicate that something went wrong. However, in many cases the caller knows that an error is highly improbable and doesn't want extra code to deal with the common case. For most methods, Golog offers one with a trailing underscore, like ByName_(), which panics on error instead of returning an error value.

See also:

* Golog's architecture: https://github.com/mndrix/golog/blob/master/doc/architecture.md
* Built in and foreign predicates: See func Builtin...
* Standard library: See golog/prelude package


Package Files

builtin.go choice_point.go clauses.go database.go foreignreturn.go interactive.go machine.go


var CutBarrierFails error = fmt.Errorf("Cut barriers never succeed")
var EmptyConjunctions = fmt.Errorf("Conjunctions list is empty")

EmptyConjunctions error is returned when trying to pop a conjunction off an empty conjunction stack. This is mostly useful for those hacking on the interpreter.

var EmptyDisjunctions = fmt.Errorf("Disjunctions list is empty")

EmptyDisjunctions error is returned when trying to pop a disjunction off an empty disjunction stack. This is mostly useful for those hacking on the interpreter.

var MachineDone = fmt.Errorf("Machine can't step any further")

MachineDone error is returned when a Golog machine has been stepped as far forward as it can go. It's unusual to need this variable.

var NoBarriers = fmt.Errorf("There are no cut barriers")

NoBarriers error is returned when trying to access a cut barrier that doesn't exist. See MostRecentCutBarrier

func BarrierId Uses

func BarrierId(cp ChoicePoint) (int64, bool)

If cp is a cut barrier choice point, BarrierId returns an identifier unique to this cut barrier and true. If cp is not a cut barrier, the second return value is false. BarrierId is mostly useful for those hacking on the interpreter or doing strange control constructs with foreign predicates. You probably don't need this. Incidentally, !/0 is implemented in terms of this.

func RegisterHelp Uses

func RegisterHelp(m Machine, h map[string]string)

type ChoicePoint Uses

type ChoicePoint interface {
    // Follow produces a new machine which sets out to explore a
    // choice point.
    Follow() (Machine, error)

ChoicePoint represents a place in the execution where we had a choice between multiple alternatives. Following a choice point is like making one of those choices.

The simplest choice point simply tries to prove some goal; usually the body of a clause. We can imagine more complex choice points. Perhaps when the choice point was created, we spawned a goroutine to investigate that portion of the execution tree. If that "clone" finds something interesting, it can send itself down a channel. In this case, the ChoicePoint's Follow() method would just return that speculative, cloned machine. In effect it's saying, "If you want to pursue this execution path, don't bother with the computation. I've already done it"

One can imagine a ChoicePoint implementation which clones the Golog machine onto a separate server in a cluster. Following that choice point waits for the other server to finish evaluating its execution branch.

func NewCutBarrier Uses

func NewCutBarrier(m Machine) ChoicePoint

NewCutBarrier creates a special choice point which acts as a sentinel value in the Golog machine's disjunction stack. Attempting to follow a cut barrier choice point panics.

func NewHeadBodyChoicePoint Uses

func NewHeadBodyChoicePoint(m Machine, g, t term.Term) ChoicePoint

A head-body choice point is one which, when followed, unifies a goal g with the head of a term t. If unification fails, the choice point fails. If unification succeeds, the machine tries to prove t's body.

func NewSimpleChoicePoint Uses

func NewSimpleChoicePoint(m Machine, g term.Term) ChoicePoint

Following a simple choice point makes the machine start proving goal g. If g is true/0, this choice point can be used to revert back to a previous version of a machine. It can be useful for building certain control constructs.

type Database Uses

type Database interface {
    // Asserta adds a term to the database at the start of any existing
    // terms with the same name and arity.
    Asserta(Term) Database

    // Assertz adds a term to the database at the end of any existing
    // terms with the same name and arity.
    Assertz(Term) Database

    // Candidates() returns a list of clauses that might unify with a term.
    // Returns error if no predicate with appropriate
    // name and arity has been defined.
    Candidates(Term) ([]Term, error)

    // Candidates_() is like Candidates() but panics on error.
    Candidates_(Term) []Term

    // ClauseCount returns the number of clauses in the database
    ClauseCount() int

    // String returns a string representation of the entire database
    String() string

Database is an immutable Prolog database. All write operations on the database produce a new database without affecting the previous one. A database is a mapping from predicate indicators (foo/3) to clauses. The database may or may not implement indexing. It's unusual to interact with databases directly. One usually calls methods on Machine instead.

func NewDatabase Uses

func NewDatabase() Database

NewDatabase returns a new, empty database

type ForeignPredicate Uses

type ForeignPredicate func(Machine, []Term) ForeignReturn

Golog allows Prolog predicates to be defined in Go. The foreign predicate mechanism is implemented via functions whose type is ForeignPredicate.

type ForeignReturn Uses

type ForeignReturn interface {
    IsaForeignReturn() // useless method to restrict implementations

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.

func BuiltinAtomCodes2 Uses

func BuiltinAtomCodes2(m Machine, args []term.Term) ForeignReturn

atom_codes/2 see ISO §8.16.5

func BuiltinAtomNumber2 Uses

func BuiltinAtomNumber2(m Machine, args []term.Term) (ret ForeignReturn)

atom_number/2 as defined in SWI-Prolog

func BuiltinCall Uses

func BuiltinCall(m Machine, args []term.Term) ForeignReturn


func BuiltinComma Uses

func BuiltinComma(m Machine, args []term.Term) ForeignReturn


func BuiltinCut Uses

func BuiltinCut(m Machine, args []term.Term) ForeignReturn


func BuiltinCutTo Uses

func BuiltinCutTo(m Machine, args []term.Term) ForeignReturn


An internal system predicate which might be removed at any time in the future. It cuts all disjunctions on top of a specific cut barrier.

func BuiltinDowncaseAtom2 Uses

func BuiltinDowncaseAtom2(m Machine, args []term.Term) ForeignReturn

downcase_atom(+AnyCase, -LowerCase)

Converts the characters of AnyCase into lowercase and unifies the lowercase atom with LowerCase.

func BuiltinFail Uses

func BuiltinFail(m Machine, args []term.Term) ForeignReturn


func BuiltinFindall3 Uses

func BuiltinFindall3(m Machine, args []term.Term) ForeignReturn


func BuiltinGround Uses

func BuiltinGround(m Machine, args []term.Term) ForeignReturn


func BuiltinIfThen Uses

func BuiltinIfThen(m Machine, args []term.Term) ForeignReturn


func BuiltinIs Uses

func BuiltinIs(m Machine, args []term.Term) ForeignReturn


func BuiltinListing0 Uses

func BuiltinListing0(m Machine, args []term.Term) ForeignReturn

listing/0 This should be implemented in pure Prolog, but for debugging purposes, I'm doing it for now as a foreign predicate. This will go away.

func BuiltinMsort2 Uses

func BuiltinMsort2(m Machine, args []term.Term) ForeignReturn

msort(+Unsorted:list, -Sorted:list) is det.

True if Sorted is a sorted version of Unsorted. Duplicates are not removed.

This is currently implemented using Go's sort.Sort. The exact implementation is subject to change. I make no guarantees about sort stability.

func BuiltinNot Uses

func BuiltinNot(m Machine, args []term.Term) ForeignReturn


func BuiltinNumericEquals Uses

func BuiltinNumericEquals(m Machine, args []term.Term) ForeignReturn


func BuiltinPrintf Uses

func BuiltinPrintf(m Machine, args []term.Term) ForeignReturn

A temporary hack for debugging. This will disappear once Golog has proper support for format/2

func BuiltinSemicolon Uses

func BuiltinSemicolon(m Machine, args []term.Term) ForeignReturn


Implements disjunction and if-then-else.

func BuiltinSucc2 Uses

func BuiltinSucc2(m Machine, args []term.Term) ForeignReturn

succ(?A:integer, ?B:integer) is det.

True if B is one greater than A and A >= 0.

func BuiltinTermEquals Uses

func BuiltinTermEquals(m Machine, args []term.Term) ForeignReturn


func BuiltinTermGreater Uses

func BuiltinTermGreater(m Machine, args []term.Term) ForeignReturn


func BuiltinTermGreaterEquals Uses

func BuiltinTermGreaterEquals(m Machine, args []term.Term) ForeignReturn


func BuiltinTermLess Uses

func BuiltinTermLess(m Machine, args []term.Term) ForeignReturn


func BuiltinTermLessEquals Uses

func BuiltinTermLessEquals(m Machine, args []term.Term) ForeignReturn


func BuiltinTermNotEquals Uses

func BuiltinTermNotEquals(m Machine, args []term.Term) ForeignReturn


func BuiltinUnify Uses

func BuiltinUnify(m Machine, args []term.Term) ForeignReturn


func BuiltinVar1 Uses

func BuiltinVar1(m Machine, args []term.Term) ForeignReturn

var(?X) is semidet.

True if X is a variable.

func ForeignFail Uses

func ForeignFail() ForeignReturn

ForeignFail indicates a foreign predicate that failed

func ForeignTrue Uses

func ForeignTrue() ForeignReturn

ForeignTrue indicates a foreign predicate that succeeds deterministically

func ForeignUnify Uses

func ForeignUnify(ts ...term.Term) ForeignReturn

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 InteractiveApropos1 Uses

func InteractiveApropos1(m Machine, args []term.Term) ForeignReturn

func InteractiveHelp0 Uses

func InteractiveHelp0(m Machine, args []term.Term) ForeignReturn

func InteractiveHelp1 Uses

func InteractiveHelp1(m Machine, args []term.Term) ForeignReturn

type Machine Uses

type Machine interface {
    // A Machine is an acceptable return value from a foreign predicate
    // definition.  In other words, a foreign predicate can perform low-level
    // manipulations on a Golog machine and return the result as the new
    // machine on which future execution occurs.  It's unlikely that you'll
    // need to do this.

    // Temporary.  These will eventually become functions rather than methods.
    // All three accept Prolog terms as strings or as io.Reader objects from
    // which Prolog terms can be read.
    CanProve(interface{}) bool
    Consult(interface{}) Machine
    ProveAll(interface{}) []Bindings

    String() string

    // Bindings returns the machine's most current variable bindings.
    // This method is typically only used by ChoicePoint implementations
    Bindings() Bindings

    // SetBindings returns a new machine like this one but with the given
    // bindings
    SetBindings(Bindings) Machine

    // PushConj returns a machine like this one but with an extra term
    // on front of the conjunction stack
    PushConj(Callable) Machine

    // PopConj returns a machine with one less item on the conjunction stack
    // along with the term removed.  Returns err = EmptyConjunctions if there
    // are no more conjunctions on that stack
    PopConj() (Callable, Machine, error)

    // ClearConjs replaces the conjunction stack with an empty one
    ClearConjs() Machine

    // ClearDisjs replaces the disjunction stack with an empty one
    ClearDisjs() Machine

    // DemandCutBarrier makes sure the disjunction stack has a cut barrier
    // on top.  If not, one is pushed.
    // This marker can be used to locate which disjunctions came immediately
    // before the marker existed.
    DemandCutBarrier() Machine

    // MostRecentCutBarrier returns an opaque value which uniquely
    // identifies the most recent cut barrier on the disjunction stack.
    // Used with CutTo to remove several disjunctions at once.
    // Returns NoBarriers if there are no cut barriers on the disjunctions
    // stack.
    MostRecentCutBarrier() (int64, error)

    // CutTo removes all disjunctions stacked on top of a specific cut barrier.
    // It does not remove the cut barrier itself.
    // A barrier ID is obtained from MostRecentCutBarrier.
    CutTo(int64) Machine

    // PushDisj returns a machine like this one but with an extra ChoicePoint
    // on the disjunctions stack.
    PushDisj(ChoicePoint) Machine

    // PopDisj returns a machine with one fewer choice points on the
    // disjunction stack and the choice point that was removed.  Returns
    // err = EmptyDisjunctions if there are no more disjunctions on
    // that stack
    PopDisj() (ChoicePoint, Machine, error)

    // RegisterForeign registers Go functions to implement Golog predicates.
    // When Golog tries to prove a predicate with one of these predicate
    // indicators, it executes the given function instead.
    // Calling RegisterForeign with a predicate indicator that's already
    // been registered replaces the predicate implementation.
    RegisterForeign(map[string]ForeignPredicate) Machine

    // Step advances the machine one "step" (implementation dependent).
    // It produces a new machine which can take the next step.  It might
    // produce a proof by giving some variable bindings.  When the machine
    // has done as much work as it can do, it returns err=MachineDone
    Step() (Machine, Bindings, error)

Golog users interact almost exclusively with a Machine value. Specifically, by calling one of the three methods Consult, CanProve and ProveAll. All others methods are for those hacking on the interpreter or doing low-level operations in foreign predicates.

func NewBlankMachine Uses

func NewBlankMachine() Machine

NewBlankMachine creates a new Golog machine without loading the standard library (prelude)

func NewInteractiveMachine Uses

func NewInteractiveMachine() Machine

func NewMachine Uses

func NewMachine() Machine

NewMachine creates a new Golog machine. This machine has the standard library already loaded and is typically the way one obtains a machine.


lexTokenize UTF-8-encoded Prolog text.
preludeDefines the Prolog standard library.
readRead Prolog terms.
termRepresent and unify Prolog terms.

Package golog imports 14 packages (graph) and is imported by 2 packages. Updated 2018-08-08. Refresh now. Tools for package owners. This is an inactive package (no imports and no commits in at least two years).