Simple interpreter

Introduction

This simple interpreter was written as student assignment.

In this project I've prepared a working interpreter of language described in project's requirements. Built program allows running (interpreting) prepared file with source code. More complex instructions (if-else, while-do, etc.) work as expected.

Required tools

I've created this project using Flex (v2.6.4), Bison (v3.7.4) and g++ (v9.3.0).

In project, I've used C++20. To use it you can compile project with -std=c+2a flag.

To simplify building the interpreter I've prepared simple Makefile.

Usage

To build the interpreter you can just use added Makefile (it's prepared for Linux).

To generate all required files and finally compile the project just run:

make

Created executable will be called interpreter.

To interpret a file with source code run pass it as argument to created executable:

./interpreter <source file name>

You can also clean all generated/compiled files by:

make clean

Files structure

Solution is split into multiple files:

• scanner-specification.l scanner specification, contains all used symbols/keywords. Parser works by just tokens to recognize keywords, their definitions are in scanner specification itself.

• parser.y parser specification.

• types.h header containing all typedefs used in the project (e.g.: type number – signed long int).

• variables.h header containing class storing all variables used by interpreted program. It also contains methods for adding and reading variables.

• callbacks.h header containing callbacks used for creating program's execution tree.

• Makefile

• test.file example file with source code

• Other files are generated when project is being built.

The way it works

Interpreter's way of working is based on callbacks. I've prepared two callback types:

• action representing some operation to be executed, which doesn't require arguments and doesn't return value. An example can be if-else construction, where, depending on condition, on of two action will be executed. This callback type is used as attributes of nonterminals which are associated with operations and not with variables or values.

• valueEval operation used for evaluating a value of some type, either variable or literal. Values also have to be handled by callbacks, as parsing is too early - program could evaluate attributes in blocks of code which are limited by conditions.

During parsing interpreter is preparing a tree of these callbacks. This tree is actually just callbacks nested within each other.

Program is being interpreted by just executing root callback, which will execute all nested callbacks.

Variable types

Number

Number type is based on signed long int in decimal format.

When user provides larger value it will be limited to either LONG_MAX or LONG_MIN.

During mathematical operations number regular overflows or underflows can happen.

String

String is a string of characters between ".

All characters are allowed, except ", \n and \r.