PROG2006 - Interpreter

Description

This is one of two assignments in the course Advanced Programming at NTNU Gjøvik. The assignment was to implement an interpreter of a high level scripting language with syntax and functionality not too dissimilar from the functional programming language Haskell, along with a CLI tool allowing interacting with the language through declaring variables, inputting lines of code and using commands to get information about types and function signatures.

Use

Demo

For a quick demo using test_program.txt, run the executable with --src="test_program.txt".

Program Arguments

• -r | --repl-mode - Runs the interpreter in repl-mode, where user input is continuously parsed and evaluated.
• -h | --help - prints information about interpreter usage to console.
• -dbg | --debug - prints tokens of a loaded file before starting executing it
• -i | --indo - Prints information about repl-mode usage
• --src="<filename>" - executes file. Assumes a plain text file with appropriate syntax.

REPL

While in REPL mode, you can continuously put new values or expressions on the stack, run functions on stack values, or print information about what's on the stack.

Commands

• :i - Prints the type and value of the top element currently on the stack.
• :h - Prints list of commands and their use
• :c - Clears the stack
• :dbg - Toggles debug mode. While debugging, all contents of the stack will be printed.
• :q - Ends REPL mode and exits the application.

Syntax

The language is stack based, meaning functions grab their arguments from a stack. This means that generally you will see postfix like notation. The snippet below puts two numbers onto the stack, runs the function + on them and then puts the result back on the stack.

> 5 5 +
> 10

Types

TypeClasses

The interpreter uses a type system based on Haskell where functions are defined on TypeClasses, classes that implement some trait compatible with the function.

Strings

Numbers

Quotation

A quotation is a segment of code delimited by {}. Can be thought of as anonymous functions that evaluate to the contents when used with higher order functions. Below, a quotation is put onto the stack, then executed with exec, leaving 10 on the stack.

bprog > { 5 5 + }
bprog > exec
bprog > :i
                Type:  Integer 
                Value: 10

Native Functions

Higher Order Functions

• map <func> - takes a list and applies func to it.

bprog > [ 1 2 3 4 ] map { 1 + }
stack > [ 2 3 4 5 ]

• each <func> - Takes a list and applies func to each element.

bprog > [ " count down! " 3 2 1 " liftoff! " ] each print
output> "count down!"
output> 3
output> 2 
output> 1
output> "liftoff!"

• exec - Executes a function from the top of the stack

• foldl <(Any, Any ->Any)> - Takes a list and a starting element with a binary function, reducing the list to a final value.

bprog > [ 1 2 3 ] 10 foldl { + } 
stack > 16

Math

Func(Num, Num -> Num)

• + - addition
• - - subtraction
• / - floating point division
• * - multiplication
• div - integer division

Func(Integer, Integer -> Integer)

• % - modulo

Logic

Func(Boolean, Boolean -> Bool)

• && - Logical AND
• || - Logical OR

Func(Num -> Num)

• not - Negation. Works on numbers and booleans.

Comparisons

Func(Ord, Ord -> Bool

• <
• >
• ==

Stack Operations

• pop - removes the top element of the stack
• swap - swaps the top elements of the stack
• dup - duplicates the top element of the stack

Defining Functions