Interpreter

Author: Colin Vandenhof
©2017 MIT License

Overview

This interpreter interprets simple scripts given in plain-text files. It was written in Prolog for CSCI 3136 Principles of Programming Languages, Winter 2017. SWI-Prolog is required to use the interpreter.

Features

• Example script, sample.txt:

int add ( int a , int b ) = a + b 
int subtract ( int a , int b ) = a - b 
int letin ( int a ) = let b = 10 in add ( a , b ) 
int equal ( int a , int b ) = if a == b then letin ( a ) else subtract ( a , 199 ) 
int main ( int input ) = equal ( input , 2 ) 

• Scripts are given in plain-text files, with space-delimited tokens.
• Scripts can contain multiple function definitions, but the interpreter always returns the output from main
• Supports integer and Boolean values for function arguments and return values
• Supports if-else conditional statements and let-in statements.
• Arithmetic operators are implemented for:
  • addition (+)
  • subtraction (-)
• Relational operators are implemented for:
  • equal to (==)
  • not equal to (!=)
  • greater than (>)
  • greater than or equal to (>=)
• The example script shows the basic syntax for function definitions.

Execution

1. In the command line, navigate to the directory containing the interpreter and the script. 2. Run SWI-Prolog

swipl

2. Load the interpreter

[language_executor].

3. Run the script

run_program(<filename>, <input arguments>, Result).

where is the name of the script (eg. 'sample.txt'), is a list of arguments for main (eg. [1]), and Result is the variable that is unified to the output of main. If the script is not valid, or invalid arguments are given, then false is returned.

Source code

• tokenizer.pl is used to read in the space-delimited script and create a list of tokens.
• lexer.pl converts the list of tokens to a list of token types (lexed list)
• grammar.pl specifies a definite clause grammar that defines the scripting language.
• parser.pl converts the lexed list into a structured (bracketed) list, based on the definite clause grammar. Indentifiers and values are recovered from the token list, and injected into the structured list.
• symmbol_table.pl defines predicates to add and remove symbols from the global symbol table.
• interpreter.pl takes in the structured list and returns the output of main.
• language_executor.pl contains the run_program/3 predicate. This subgoals of this predicate are all of the steps in the pipeline (from tokenizer to interpreter).