I had previously discussed how one can implement a programming language using big step semantics. In this post, I want to so something similar. Here, we implement a meta-circular interpreter over Python. The code is available here.

A meta-circular interpreter is an interpreter for a language that is written in that language itself. The MCI can implement a subset or superset of the host language.

## Uses of a Meta Circular Interpreter

Why should one want to write a meta-circular interpreter? Writing such an interpreter gives you a large amount of control over how the code is executed. Further, it also gives you a way to track the execution as it proceeds. Using the same machinery as the meta-circular interpreter, one can:

• Write a concolic interpreter that tracks the concrete execution
• Extract coverage
• Extract the control flow graph, and execution path through the graph
• Extract the call graph of execution
• Write a symbolic execution engine
• Write a taint tracker that will not be confused by indirect control flow
• Extend the host language with new features
• Reduce the capabilities exposed by the host language (e.g. no system calls).

I will be showing how to do these things in the upcoming posts.

## The Implementation (tested in Python 3.6.8)

First, we import everything we need.

Important: Pyodide takes time to initialize. Initialization completion is indicated by a red border around Run all button.

The basic idea is to make use of the Python infrastructure as much as possible. That is, we do not want to implement things that are not related to the actual interpretation. Hence, we use the Python parsing infrastructure exposed by the ast module that parses Python source files, and returns back the AST. AST (Abstract Syntax Tree) as the name indicates is a data structure in the format of a tree.

Once we have the AST, we simply walk the tree, and interpret the statements as we find them.

### The meta-circular-interpreter class

The walk() method is at the heart of our interpreter. Given the AST, It iterates through the statements, and evaluates each by invoking the corresponding method. If the method is not implemented, it raises a SynErr which is derived from SyntaxError.

We provide eval() which converts a given string to its AST, and calls walk(). It is possible to write a parser of our own, as I have shown before which can get us the AST. However, as I mentioned earlier, we use the Python infrastructure where possible.

#### The Pythonic data structures.

We need to define data. For the primitive data types, we only implement string and number for now. These are trivial as it is a direct translation of the AST values.

#### Containers

Essentially, we want to be able to make use of all pythonic container data structures such as lists, tuples, sets and dictionaries. For demonstration, however, we have implemented only list and tuple.

Unlike the primitives, the containers may be defined such that the values inside them are result of evaluating some expression. Hence, we first walk() the elements, and add the results.

##### Tuple(elts)

Containers provide the ability to access their contained items via Subscript.

##### Subscript(expr value, slice slice, expr_context ctx)

The tuple and list provide a means to access its elements via subscript. The subscript requires a special Index value as input, which is also defined below.

##### Attribute(expr value, identifier attr, expr_context ctx)

Similar to subscript for arrays, objects provide attribute access.

#### Simple control flow statements

The return, break and continue are implemented as exceptions.

#### Implementation

The difference between break and continue is in how they are handled in the loop statemens as in While below. The return is handled in the Call part.

#### Major control flow statements

Only basic loops and conditionals – while() and if() are implemented.

##### While(expr test, stmt* body, stmt* orelse)

Implementing the While loop is fairly straight forward. The while.body is a list of statements that need to be interpreted if the while.test is True. The break and continue statements provide a way to either stop the execution or to restart it.

As can be seen, these are statements rather than expressions, which means that their return value is not important. Hence, we do not return anything.

##### If(expr test, stmt* body, stmt* orelse)

The If statement is similar to While. We check if.test and if True, execute the if.body. If False, we execute the if.orelse.

#### The scope and symbol table

Now we come to a slightly more complex part. We want to define a symbol table. The reason this is complicated is that the symbol table interacts with the scope, which is a nested data structrue, and we need to provide a way to look up symbols in enclosing scopes. We have a choice to make here. Essentially, what variables do the calling program have access to? Historically, the most common conventions are lexical and dynamic scoping. The most intuitive is the lexical scoping convention. Hence, we implement lexical scoping, but with a restriction: If we modify a variable in parent scopes, then the new variable is created in current scope.

#### Hooking up the symbol table

We allow the user to load a pre-defined symbol table. We have a choice to make here. Should we allow access to the Python default symbol table? and if we do, what should form the root? The Python symbol table or what the user supplied?

Here, we assume that the default Python symbol table is the root.

We will discuss the OP statements later.

#### The following statements use symbol table.

##### Name(identifier id, expr_context ctx)

Retrieving a referenced symbol is simple enough.

##### Assign(expr* targets, expr value)

Python allows multi-target assignments. The problem is that, the type of the value received may be different based on whether the statement is multi-target or single-target. Hence, we split both kinds.

##### Call(expr func, expr* args, keyword* keywords)

During function calls, we need to make sure that the functions that are implemented in C are proxied directly.

For others, we want to correctly bind the arguments and create a new scope. The big question is how should the scopes be nested. We use lexical scopes. So, we recover the symbol table used at the time of definition, use it for the call, and reset it back to the current one after the call.

Note that we handle the return exception here.

##### FunctionDef(identifier name, arguments args, stmt* body, expr* decorator_list, expr? returns, string? type_comment)

The function definition itself is quite simple. We simply update the symbol table with the given values. Note that because we implement lexical scoping, we have to maintain the scoping references during creation.

##### Import(alias* names)

Import is similar to a definition except that we want to update the symbol table with predefined values.

#### Arithmetic Expressions

The arithmetic expressions are proxied directly to corresponding Python operators.

#### Modules

##### Module(stmt* body)

The complete AST is wrapped in a Module statement.

### The driver

if __name__ == '__main__':

\$ python3 interp.py triangle.py '1 2 3'