A small Pascal-like interpreter built as a learning project.

Current pipeline:

source text -> Lexer -> Parser -> AST -> SemanticAnalyser -> Interpreter

• Lexer for Pascal-like tokens (PROGRAM, VAR, BEGIN/END, arithmetic ops, assignment, literals, identifiers)
• Recursive-descent parser that builds an AST
• AST node model in nodes.py
• Visitor-based execution (NodeVisitor + Interpreter)
• Semantic pass (SemanticAnalyser) with symbol table population, duplicate declaration checks, undeclared variable checks, assignment compatibility checks, and numeric operator checks
• Script runner supports startup file arguments and interactive :run <path> execution

• src/main.py: entry point and script command loop
• src/Lexer.py: lexical analysis
• src/Parser.py: AST construction from tokens
• src/nodes.py: AST node classes
• src/interpreter.py: base visitor and runtime interpreter
• src/SemanticAnalyser.py: semantic checks + symbol table population
• src/tokens.py: token constants, token class, symbol classes/table, and custom exceptions
• grammar.txt: grammar notes
• instructions/instructions.txt: sample input program
• instructions/a.pas: additional sample script

program : PROGRAM variable SEMI block DOT

block : declarations compound_statement

declarations : VAR (variable_declaration SEMI)+
             | empty

variable_declaration : ID (COMMA ID)* COLON type_spec

type_spec : INTEGER | REAL

compound_statement : BEGIN statement_list END

statement_list : statement
               | statement SEMI statement_list

statement : compound_statement
          | assignment_statement
          | empty

assignment_statement : variable ASSIGN expr

empty :

expr : term ((PLUS | MINUS) term)*

term : factor ((MUL | INTEGER_DIV | FLOAT_DIV) factor)*

factor : PLUS factor
       | MINUS factor
       | INTEGER_CONST
       | REAL_CONST
       | LPAREN expr RPAREN
       | variable

variable : ID

Note: standalone expression statements like 1+1 are not valid in this grammar.

1. Use Python 3.
2. From the project folder:

python src/main.py

Behavior:

• Pass a script path as an argument to run immediately:

python src/main.py instructions/instructions.txt

• Or start interactive mode and run scripts by path:

script> :run instructions/instructions.txt

• You can also type a path directly at the prompt:

script> instructions/a.pas

• Type :q to quit

• Variables must be declared before use
• Duplicate variable declarations are rejected
• DIV requires INTEGER operands
• / (FLOAT_DIV) yields REAL
• +, -, * yield INTEGER only when both operands are INTEGER, otherwise REAL
• Assignments allow exact type match and widening INTEGER -> REAL

The project currently uses:

• LexerError
• ParserError
• InterpreterError (also used by semantic analysis at the moment)

• No procedures/functions yet
• No nested scopes yet
• No booleans/relational operators yet
• Semantic errors are not split into a dedicated SemanticError type yet