This library provides a Logtalk port of the Peter Van Roy’s extended DCG implementation. For full documentation on EDCGs, see:


This Logtalk version defines a hook object, edcg. Source files defining EDCGs must be compiled using the compiler option hook(edcg):

| ?- logtalk_load(source, [hook(edcg)]).

Alternatively, the following directive can be added at the beginning of the source file containing the EDCGs:

:- set_logtalk_flag(hook, edcg).

The hook object automatically adds the EDCGs -->> infix operator scoped to the source file.

This port has simplified by copying and then modifying Michael Hendricks’s edcg repo at:


A notable difference is that Michael’s version declares Peter’s original predicates for declaring accumulators and predicates using the hidden arguments as multifile predicates. But this is risky as two independent EDCGs may use e.g. the same accumulator names and introduce conflicts. The Logtalk version uses instead the edcg hook object internal state to temporarily save those predicates in order to parse the corresponding EDCGs.

API documentation

Open the ../../docs/library_index.html#edcg link in a web browser.


To load all entities in this library, load the loader.lgt utility file:

| ?- logtalk_load(edcg(loader)).


To test this library predicates, load the tester.lgt file of the edcgs example:

| ?- logtalk_load(edcgs(tester)).


Follows the usage documentation written by Michael Hendricks (with a contribution from Peter Ludemann), used here with permission, with the necessary changes for the Logtalk port.

% declare accumulators
acc_info(adder, X, In, Out, integer::plus(X,In,Out)).

% declare predicates using these hidden arguments

increment -->>
    % add one to the accumulator

len(Xs,N) :-

len -->>
    % 'dcg' accumulator has an element
    % increment the 'adder' accumulator
len -->>


DCG notation gives us a single, hidden accumulator. Extended DCG notation (implemented by this library) lets predicates have arbitrarily many hidden accumulators. As demonstrated by the synopsis above, those accumulators can be implemented with arbitrary goals (like integer::plus/3).

Benefits of this library:

  • avoid tedium and errors from manually threading accumulators through your predicates

  • add or remove accumulators with a single declaration

  • change accumulator implementation with a single declaration (ex, switching from ordsets to rbtrees)


Extended DCG syntax is very similar to DCG notation. An EDCG is created with clauses whose neck is the -->> operator. The following syntax is supported inside an EDCG clause:

  • {Goal} - don’t expand any hidden arguments of Goal

  • Goal - expand all hidden arguments of Goal that are also in the head. Those hidden arguments not in the head are given default values.

  • Goal:L - If Goal has no hidden arguments then force the expansion of all arguments in L in the order given. If Goal has hidden arguments then expand all of them, using the contents of L to override the expansion. L is either a term of the form Acc, Acc(Left,Right), Pass, Pass(Value), or a list of such terms. When present, the arguments Left, Right, and Value override the default values of arguments not in the head.

  • List:Acc - Accumulate a list of terms in the accumulator Acc

  • List - Accumulate a list of terms in the accumulator dcg

  • X/Acc - Unify X with the left term for the accumulator Acc

  • Acc/X - Unify X with the right term for the accumulator Acc

  • X/Acc/Y - Unify X with the left and Y with the right term for the accumulator Acc

  • insert(X,Y):Acc - Insert the arguments X and Y into the chain implementing the accumulator Acc. This is useful when the value of the accumulator changes radically because X and Y may be the arguments of an arbitrary relation

  • insert(X,Y) - Insert the arguments X and Y into the chain implementing the accumulator dcg. This inserts the difference list X-Y into the accumulated list

Declaration of Predicates

Predicates are declared with facts of the following form:

pred_info(Name, Arity, List).

The predicate Name/Arity has the hidden parameters given in List. The parameters are added in the order given by List and their names must be atoms.

Declaration of Accumulators

Accumulators are declared with facts in one of two forms. The short form is:

acc_info(Acc, Term, Left, Right, Joiner).

The long form is:

acc_info(Acc, Term, Left, Right, Joiner, LStart, RStart).

In most cases the short form gives sufficient information. It declares the accumulator Acc, which must be an atom, along with the accumulating function, Joiner, and its arguments Term, the term to be accumulated, and Left & Right, the variables used in chaining.

The long form of acc_info is useful in more complex programs. It contains two additional arguments, LStart and RStart, that are used to give default starting values for an accumulator occurring in a body goal that does not occur in the head. The starting values are given to the unused accumulator to ensure that it will execute correctly even though its value is not used. Care is needed to give correct values for LStart and RStart. For DCG-like list accumulation both may remain unbound.

Two conventions are used for the two variables used in chaining depending on which direction the accumulation is done. For forward accumulation, Left is the input and Right is the output. For reverse accumulation, Right is the input and Left is the output.

Declaration of Passed Arguments

Passed arguments are conceptually the same as accumulators with =/2 as the joiner function. Passed arguments are declared as facts in one of two forms. The short form is:


The long form is:

pass_info(Pass, PStart).

In most cases the short form is sufficient. It declares a passed argument Pass, that must be an atom. The long form also contains the starting value PStart that is used to give a default value for a passed argument in a body goal that does not occur in the head. Most of the time this situation does not occur.

Additional documentation

Peter Van Roy’s page: Declarative Programming with State

Technical Report UCB/CSD-90-583 Extended DCG Notation: A Tool for Applicative Programming in Prolog by Peter Van Roy

  • The Tech Report’s PDF is here

A short Wikipedia article on DCGs and extensions.