Pascal.js

Description

Pascal.js is a Pascal compiler (Turbo Pascal 1.0-ish) implemented in JavaScript that outputs LLVM IR (intermediate representation). The IR can then compiled to native machine code (using LLVM as a backend), or compiled to JavaScript (via LLVM.js) so that it can run as a Node.js script or can run in a browser.

Pascal.js uses an LALR(1) (bottom-up lookahead left-to-right) parser generated using Jison.

Pascal.js is written in JavaScript and outputs JavaScript. This means you can write, compile and execute Pascal programs entirely in your browser. Try out the obligatory online demo of Pascal.js (supports at least Chrome and Firefox).

Why?

Take your pick:

• It has been said: "Great programmers don't program in Pascal but they have built a compiler for it." Turbo Pascal was my first programming language (or it could have been Turtle Logo) and now I have built a compiler for it. I'm not sure where that leaves me ...

OR

• Pascal and JavaScript exist. Sooner or later somebody was going to think of putting them together ...

OR

• In the Spring of 2013 I took a compiler class as part of my graduate studies. The class project was to build a compiler in Java for a simplified Pascal variant (PCAT). For many years I have been wanting to ressurect some old Turbo Pascal programs I created in grade school. There were two obvious paths available to me: get the original Turbo Pascal compiler running using DOSBox, or create a Turbo Pascal compiler from scratch (inspired by my class work). I took both paths (of course!). This is the result of that second path.

Usage

Make sure that you have the llvm.js submodule checked out:

git submodule init
git submodule update

Download the node jison module and use it to build the lexer/parser module:

make parser.js

Compile to JavaScript using compile.js and run the resulting executable directly with node:

compile.js tests/ffib.pas build/ffib.js
node build/ffib.js

To build native executables, pascal.js uses the llc executable that is part of clang/LLVM. You can install clang on Debian/Ubuntu like this:

sudo apt-get install clang

Compile to a native executable using compile_native.js and run the resulting executable:

compile_native.js tests/ffib.pas build/ffib
build/ffib

Generate LLVM IR (skipping assembly) using ir.js like this:

node ir.js tests/ffib.pas > build/ffib.ll

Language Features Supported

The following features of Turbo Pascal 1.0 are supported:

• Procedures and functions with nested lexical scope
• Scalar Types: Integer, Real, Character, Byte, Boolean, Enumeration, Subrange
• Structured Types: String, Array (multidimensional), Record, Enumeration
• User defined types
• Automatic and explicit type transfer (coercion): Integer(), Char()
• Forward references (for corecursion)
• Control statements: if/else, for, while, repeat, case
• Standard routines: Concat, Chr, Halt, Odd, Ord, Random, Read, ReadLn, Write, WriteLn
• Crt routine (native and Node.js only): ClrScr, Delay, GotoXY, KeyPressed, ReadKey

Missing Language Features / Caveats

The following features of Turbo Pascal 1.0 are not yet supported:

• Pointer type
• Set type
• With statement
• Typed constants
• File types / file routines
• Untyped variable parameters
• Additional standard/crt routines

Some other differences from Turbo Pascal 1.0:

• Crt unit must be explicityly specified
• Default case branch is specified with "otherwise" rather than "else" (to address a parsing ambiguity with if/else).
• Strings are current C-style (null terminated). Also, string assignment and concatentation leaks memory.

Intentionally Omitted Language Features

• Labels / Goto
• Compiler directives
• Mem and Port built-in arrays
• Variant Records