A study in interpreters

Based on this wonderful series of articles:

Two implementations:

One in Python, following Ruslan's example
One in Rust (because I want to learn how to build a compiler and learn rust at the same time.)

To run the rust compiler:

cargo run program27.pas

To debug, change the log levels in parser.rs or in node_visitor.rs.

Explanation

lexer: reads the file and turns the text into tokens
parser: contains the definitions of the AstNodes (Abstract Syntax Tree Nodes)
node_visitor: contains the implementations to walk the various AstNodes
interpreter: initiates the tree walking to execute/evaluate the program
datum: defitions of return types, result of evaluation nodes
call_stack: definition of the call stack + functions to access, push, pop and insert
symbols: definitions of symbols (for semantic analysis)
scope: call stack for semantic analysis
semantic_analyzer: walks the program tree to perform semantic analysis (i.e. not syntax errors, but semantic errors)
main: entry point
lib: program