

grammar::fa::dexec(n) 0.2 tcllib "Finite automaton operations and usage"


grammar::fa::dexec - Execute deterministic finite automatons

Table Of Contents



This package provides a class for executors constructed from deterministic finite automatons (DFA). Executors are objects which are given a string of symbols in a piecemal fashion, perform state transitions and report back when they enter a final state, or find an error in the input. For the actual creation of the DFAs the executors are based on we have the packages grammar::fa and grammar::fa::op.

The objects follow a push model. Symbols are pushed into the executor, and when something important happens, i.e. error occurs, a state transition, or a final state is entered this will be reported via the callback specified via the option -command. Note that conversion of this into a pull model where the environment retrieves messages from the object and the object uses a callback to ask for more symbols is a trivial thing.

Side note: The acceptor objects provided by grammar::fa::dacceptor could have been implemented on top of the executors provided here, but were not, to get a bit more performance (we avoid a number of method calls and the time required for their dispatch).


The package exports the API described here.

::grammar::fa::dexec daName fa ?-any any? ?-command cmdprefix?

Creates a new deterministic executor with an associated global Tcl command whose name is daName. This command may be used to invoke various operations on the executor. It has the following general form:

daName option ?arg arg ...?

Option and the args determine the exact behavior of the command. See section EXECUTOR METHODS for more explanations.

The executor will be based on the deterministic finite automaton stored in the object fa. It will keep a copy of the relevant data of the FA in its own storage, in a form easy to use for its purposes. This also means that changes made to the fa after the construction of the executor will not influence the executor.

If any has been specified, then the executor will convert all symbols in the input which are unknown to the base FA to that symbol before proceeding with the processing.


All executors provide the following methods for their manipulation:

daName destroy

Destroys the automaton, including its storage space and associated command.

daName put symbol

Takes the current state of the executor and the symbol and performs the appropriate state transition. Reports any errors encountered via the command callback, as well as entering a final state of the underlying FA.

When an error is reported all further invokations of put will do nothing, until the error condition has been cleared via an invokation of method reset.

daName reset

Unconditionally sets the executor into the start state of the underlying FA. This also clears any error condition put may have encountered.

daName state

Returns the current state of the underlying FA. This allow for introspection without the need to pass data from the callback command.


The callback command cmdprefix given to an executor via the option -command will be executed by the object at the global level, using the syntax described below. Note that cmdprefix is not simply the name of a command, but a full command prefix. In other words it may contain additional fixed argument words beyond the command word.

cmdprefix error code message

The executor has encountered an error, and message contains a human-readable text explaining the nature of the problem. The code on the other hand is a fixed machine-readable text. The following error codes can be generated by executor objects.


An unknown symbol was found in the input. This can happen if and only if no -any symbol was specified.


The underlying FA has no transition for the current combination of input symbol and state. In other words, the executor was not able to compute a new state for this combination.

cmdprefix final stateid

The executor has entered the final state stateid.

cmdprefix reset

The executor was reset.

cmdprefix state stateid

The FA changed state due to a transition. stateid is the new state.


Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category grammar_fa of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.


automaton, execution, finite automaton, grammar, parsing, regular expression, regular grammar, regular languages, running, state, transducer


Grammars and finite automata