Language Implementation with Rascal – A Tutorial

• Anya Helene Bagge, Department of Informatics, University of Bergen

History

• First edition: Tutorial at CoCoDo'17
• Second edition: Tutorial at UFCG in Campina Grande, Brazil, 2017
• Third edition: Tutorial at CoCoDo'18

Setup

• Install (if you don't have it already) a full Java 8 JDK. You may want to uninstall any previously installed JREs, to avoid confusion. Rascal needs a full JDK, and may fail if it's launched with just a JRE.

• Install Eclipse Oxygen.2 for RCP/RAP developers from https://www.eclipse.org/downloads/packages/eclipse-rcp-and-rap-developers/neon3 (other versions may work) (Oh, and be careful to not have spaces in your installation directory!)

• Install Rascal from http://www.rascal-mpl.org/start/

• Clone the CoCoDo Rascal source from: https://github.com/anyahelene/cocodo.git

  • In Eclipse: File → Import → Git → Projects From Git

Resources

• Lecture notes (Rascal language implementation tutorial): https://bytebucket.org/sle-uib/inf225/wiki/docs/full.pdf?rev=8426bd7d0f78856368eae5d9542c662989c9f4df

• A more advanced (full) implementation of different variants of the Simpl language: https://bitbucket.org/anyahelene/inf225public/src/b4731730e165a019aeb3ea084a5a904d1d28dc8d/Evaluator/?at=master

• Rascal help: http://www.rascal-mpl.org/help/

• Rascal library reference: http://tutor.rascal-mpl.org/Rascal/Rascal.html

Things you should (hope) to learn

• Get a brief overview of what Rascal is and what it can be used for

• Basics of syntax definition – be aware of generalised parsing and ambiguities

• Basics of working with concrete syntax

• Strings and string interpolation

• Visiting data structures

• Basics of making an IDE

Simple experiments

Open the CoCoDo project. You'll find various source code in the 'src' folder.

If you don't have a Rascal console open in Eclipse already, right click on the project and select Rascal console. Make sure the title of the console tab includes "project: cocodo".

Set up language plugin

This will setup up syntax highlighting for the languages we're creating.

rascal>import Plugin;
ok
rascal>main();
ok

Simpl Language

Import the language definition:

import simpl::Simpl;

Remember the semicolon at the end, otherwise you'll get a confusing >>> prompt!

If you want to play around with parsing, you should also import the ParseTree module:

import ParseTree;

example = parseSimpl(|project://cocodo/src/simpl/example.simpl|)

Things to do

• The minus operator is not implemented. Add it to the evaluator and typechecker.
• Simpl is half way between a language based on expressions, and a language that takes a list of definitions, and then a list of expressions. This should be fixed in the evaluator and the typechecker (it's done already for the Simpr language)
• The Plugin file defines the Eclipse plugin, have a look at it, and
• The language is missing a less-than (or other comparison) operator. Add it. Note that the less-than symbol is reserved in Rascal, so you need to escape it: \<.
• There is syntax for if, but the evaluator (and typechecker) doesn't support it. Add if.
• Priorities and associativity is wrong: make them left associative and at the same priority

More excitement

In the file src/simpl/Simplifier there's a simple constant folding transformation.

• Add support for other operators
• Try it on nested expressions (it won't work!). You need a bottom-up traversal (bottom-up visit).
• Add support for variables – you should make a recursive simplify function, make it take an environment (e.g., map[ID, int]) as argument and call itself recursively when it encounters a let.

TAC Language

We also have a language for three-address code.

import tac::ThreeAddressCode;

example = parseTACProgram(|project://cocodo/src/tac/example.tac|)

The parseProgram function is shorthand for this:

import ParseTree;
programTop = parse(#start[TacProgram], |project://cocodo/src/tac/example.tac|);
program = programTop.top;

Running the virtual machine

import tac::VirtualMachine;
executeProgram(example, "main", [])

Solutions

• If evaluator: public Value eval((Expr)if then else end, Env env) = eval( (Int(0) := eval(cond, env)) ? e2 : e1, env );