pbsm 0.3.1

Python based stack machine

This work is inspired by Sven Havemanns work on Generative Modelling. The idea is to create a stack machine in Python that is minimalistic but easy to extended. Minimalistic means that it is boiled down to its bare minimum. All the rest is down with extensions written in Python

Sven has oriented himself at the Postscript interpreter and so is this state machine. Because it is based on Python, there are a few differences to the Postscript stack machine:

• All objects on the stack are Python objects.

• Postscript uses () to denote strings. PBSM uses the standard Python syntax for strings: ', " or """ enclose a valid string.

• Postscript use `/` to push a literal on the stack. PBSM uses ' instead. This makes the syntax more consistent and uses less characters with special meaning.

Datatypes

PBSM uses standard object types of Python: int, float, string, bool and list. It introduces the following types:

• Marker to denote the start of a list/executable list.

• Symbol for a symbolic name.

• Reference references a symbol

• Procedure represents an executable list.

Commands

The core interpreter comes with a few commands only. These are:

• [ together with ] creates the a list.

• { together with } creates an executable list/a procedure.

• cvx converts a list to an executable list/procedure

• cvlit converts an executable list/procedure back to a normal/litaral list

• def assigns a value to a symbol.

• exec executes the object on the stack.

• mark puts a mark on the stack

• counttomark counts the elements on the stack up to the topmost mark

• cleartomark removes all objects from the stack up to the topmost mark but not the mark itself.

That’s it. All the rest is done with extensions.

Execution

The stack machine maintains a stack of dictionaries in which the values of the symbols are stored. Every extention can register a dictionary with name/object pairs. Callable objects are executable. The machine tries to resolve the symbol by looking in each of the dictionaries in a top-down fashen. If it finds a value, it executes the value. Execution means:

• If the object is callable, it is called.

• If it is a symbol, the symbol is looked up and the process starts again recursively.

• All other objects are put on the stack.

Examples

List creation:

[ 5.0 True "hello" ]

Executable list creation:

{ dup print }

This is syntactial sugar for:

[ 'dup 'print ] cvx

Procedure definitaion:

'average { 2 div } def