Interactive interpreter for an object-capability programming language, written in Python2 using PyMeta
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Interactive Interpreter for the Class Programming Language

The Class language is a (toy) object-capability programming language. It has formal semantics, which allows proving that programs cannot violate the object-capability properties. I developed the language and its formal (operational) semantics as part of my master's thesis as a (more) rigorous foundation for research on object-capability programming languages.

Even though the development of Class focused on theoretical studies, having an interpreter for the language was very helpful for debugging the semantics. This repository contains an interactive interpreter for Class programs written in Python2. It supports stepping through or executing programs, as well as inspecting the resulting configurations.

I provide the interpreter here as an example for (a) how such an interpreter can be implemented by hand, and (b) how to use PyMeta for parsing. If you are developing a new programming language, instead of writing everything by hand, consider using a modern semantics framework like the K framework or Spoofax. Semantics defined in these frameworks are executable, which means that they provide an interpreter for the defined language for free.

System Requirements

The interpreter is implemented in Python, a dynamic programming language available on all common platforms. To be able to run the interpreter, please install Python version 2.5 or greater on your system. Please refer to Python's own documentation for instructions specific to your platform. Also, note that Python version 3.x is partially incompatible with the 2.x series of Python. The interpreter will therefore not work with it.

Besides Python itself, the interpreter relies on the PyMeta module. PyMeta is an object-oriented pattern matcher based on OMeta and is used for parsing the Class-programs. Please ensure that it is available within Python's module path; you can modify this path via the PYTHONPATH environment variable. Again, please refer to Python's documentation for exact instructions. If everything is set up correctly, Python's interactive command shell allows importing the module via import pymeta. Otherwise you will receive an ImportError exception.

Installing and Running

No installation is required for the Class interpreter---you can start it directly from the downloaded repository. Simply execute the file


The interpreter provides an interactive shell for loading, executing and inspecting Class programs; its prompt is the string Class Interpreter>. In the text below, typewriter style denotes interaction with the interpreter.

Most of this section's documentation is available on line within the interpreter. You can use the command help to retrieve a list of available commands. To receive further information on a command c, enter help c. Issuing exit or quit will terminate the interpreter. An empty line repeats the previous command.

Note that you can speed up typing by using command and parameter completion with the Tab key if the readline Python module is installed. Hitting the Tab key twice will present you with a list of possible alternatives. Readline comes as a part of Python itself on Unix and related operating systems such as Linux and MacOS~X.

Loading Programs

A typical session with the interpreter starts with loading a program through the command load.

Syntax: load <file name>

The command loads and parses the program stored in file . It returns nothing on success, but prints an error message if the file could not be opened or the contents could not be parsed.

Please note that a limitation of the PyMeta pattern matcher reduces the helpfulness of error messages. The construct reported as causing the error only marks the beginning of the biggest part that failed. Hence, a whole construct, such as a class declaration, is tagged wrong if one of its parts contains errors, for example if a method body misses a semicolon.

Example. The interpreter comes together with an example program in the file busy.cls that simulates the 3-state busy beaver. We want to experiment with it and therefore load it into the interpreter. This results in below interaction.

Class Interpreter> load busy.cls
Class Interpreter>

Executing and Stepping Through a Program

Loaded programs can be executed with the step command. It applies the transition relation times to the current configuration.

Syntax: step [<number of steps>]

Note that it typically takes several applications of the transition relation to execute a Class statement. Therefore, is always higher than the number of statements the program advances. If the number of steps is omitted, the program executes a single step.

The command returns nothing on success and issues a note if the program terminated. The final store's contents will remain available after the program finished. However, the then-current frame contains no variables and, hence, does not allow browsing through the store. See the discussion on labelling objects for a workaround of this limitation.

Example. Suppose we knew from previous experiments that it requires 37 steps to execute all initialisation statements of the busy beaver program. Continuing above example, we fast forward to the point where the run() method is called as shown below.

Class Interpreter> step 37
Class Interpreter>

Printing the Current Configuration's Statement

How is it possible to know the number of steps it takes until a certain statement? A simple approach is to small-step through the program and check the intermediate configurations' statements with the program command.

Syntax: program

The command prints the statement of the current configuration. The code reflects all transformations that were applied during the execution up to this point.

Example. Picking up the example again, we check whether 37 steps really took us to the right statement in the program.

Class Interpreter> program
  { };
  return self
Class Interpreter> 

Note that the interpreter employs its own pretty printer to output the statements. Indentation and line-breaks will therefore possibly differ from the source file's contents if you invoke the command directly after loading a program.

Inspecting the Store

Beside the current configuration's statement, it is also possible to inspect its store. The command inspect allows looking at objects.

Syntax: inspect [--depth <depth>] <object path>+

The optional parameter specifies the inspection depth d. With d=0, only the specified object obj itself will be printed on the screen. A depth of d>0 will treat all objects that obj holds references to (in its member variables) as being specified, too, with a depth of d-1.

A list of object paths specifies which objects to inspect. An object path is a, possibly empty, string that describes a reference to an object in the store. It consists of segments joined by periods. For example, the object path one.two has two segments: one and two.

Each segment names a member variable in the object designated by the previous segment. Their combination then stands for the reference that is the value of this last member variable. In above example, path one.two denotes a reference to the object that member variable two of (the object referred to by) one points to.

Object paths always start at the current frame object pointer. Hence, the path one means the reference that is the value of the current frame's member variable one. Because scoping requires indirection, the object referred to is variable one's container. Consequently, path is the value of variable one in the current scope. See the thesis section on memory contents in Class for a detailed explanation.

Note that Tab completion also works for object paths: Pressing Tab once completes the segment under the cursor as far as the current prefix is unique; pressing it twice prints a list of valid alternatives.

Example. After having seen the current configuration's statement in the previous subsection, we have a look at some of the objects. We start by inspecting the current frame.

Class Interpreter> inspect
Object at ref:0x8a3e8cc
char_0         ->  ref:0x8a3e06c
char_1         ->  ref:0x8a3e06c
current_state  ->  ref:0x8a3e06c
false          ->  ref:0x8a3e06c
head           ->  ref:0x8a3e06c
int:CLASS      ->  ref:0x882c28c
int:PREV       ->  ref:0x8a3e10c
self           ->  ref:0x8a3e14c
state_A        ->  ref:0x8a3e06c
state_B        ->  ref:0x8a3e06c
state_C        ->  ref:0x8a3e06c
true           ->  ref:0x8a3e06c

Class Interpreter> 

Afterwards we check the cell under the Turing machine's read and write head.

Class Interpreter> inspect head.head
Object at ref:0x8a3ec0c
content          ->  ref:0x8a3eb8c
default          ->  ref:0x8a3eb8c
left_neighbour   ->  ref:0x8a3ec0c
right_neighbour  ->  ref:0x8a3ec0c
Class Interpreter> 

Internalised names are accessible through a special prefix to a segment. The segment internal:class, or i:c for short, denotes the internalised name class. Likewise, internal:previous or i:p is the internalised name prev.

Two ways to describe absolute references complete above relative path segments. The first is by label via label:foo where foo was assigned beforehand to an object path using the command label. Prefix l: is an abbreviation for label:. The second way is by memory address using the prefix reference: or r:.

Example. The command inspect i:prev.x.x lets us look at the object that is the value of variable x in the previous frame.

Labelling Objects for Later Inspection

It is often convenient to remember objects for later inspection. The command label makes it possible to tag an object denoted by an object path with a user-defined name . This makes it easy to retrieve the object, even if it would be complicated to address from within the current frame.

Syntax: label <object path> <name>

Afterwards, use the segment label:<name> to refer to the object. This form will also be used as the preferred name when looking at objects with inspect.

Set labels can be removed with the command unlabel.

Syntax: unlabel <name>

If an object path---even with a label: structure---is used as argument instead of a simple name, all labels to the specified object will be removed.

Example. Let us continue working with the busy beaver from the previous example. Our goal is to inspect the final tape contents. A possible approach is to remember the frame from the constructor and check the bound variable head after the program terminated. For better readability, we also assign names to the tape symbols 0 and 1.

Class Interpreter> label . cframe
Class Interpreter> label char_0.char_0 0
Class Interpreter> label char_1.char_1 1
Class Interpreter> step 500
The program finished execution after 386 steps.
Memory contents remain available for inspection
until a new program is loaded.
Class Interpreter> inspect l:cframe.head.head
Object at ref:0xa571e8c
content          ->  label:1
default          ->  label:0
left_neighbour   ->  ref:0xa56cc0c
right_neighbour  ->  ref:0xa5828ac

From this cell on we can use the left_neighbour and right_neighbour member variables to traverse the whole tape.


Copyright (c) 2008--2012 Peter Dinges

The software in this repository is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

The software is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see