stork

Welcome back!

For those of you just joining, Stork is an example programming language “course” designed to demonstrate the principles of programming language implementation in 10 “lessons.” This is Lesson 4 in a series of 10, so if you’re just joining now, you may want to take a peek at lessons 1, 2, and 3 to gear up a bit for this post.

Lesson 4 adds variables to Stork, which involves adding statements in addition to the expressions already in the language. The addition of variables provides fodder for some additional (and more interesting) static analysis as well. At the end of this lesson, Stork will be a working interpreter for simple numerical expressions with support for variables. (The variables will become much more interesting over the course of the next couple of lessons, which will add support for functions and control structures.)

The code for this lesson is available on github under the tag lesson4, and you can follow the discussion about this lesson on reddit. As a quick preview, though, Stork is getting cool:

$ java com.sigpwned.stork.Stork
>>> var x:Int
>>> x
ERROR: Variable may not have been initialized: x
>>> x = 1+2*(3+4)
15
>>> var y:Float
>>> y = x
15.0
>>> x = y
ERROR: Will not coerce to less precise type: Float -> Int
>>> x = (cast Int) y
15
>>> x+y
30.0
>>> ^D
$

Welcome back!

For those of you just joining, Stork is an example programming language “course” designed to demonstrate the principles of programming language implementation in 10 “lessons.” This is Lesson 3 in a series of 10, so if you’re just joining now, you may want to check out lessons 1 and 2 to gear up a bit for this post.

Lesson 3 covers the basics of compiler design (front end versus back end) and types, plus a very brief preview of static analysis. At the end of this lesson, Stork will be a working interpreter for simple numerical expressions.

The code for this lesson is available on github under the tag lesson3, and you can follow the discussion about this lesson on reddit.

(Basic) Compiler Theory

Most developers are familiar with the use of compilers like javac and gcc — instant program, just add source code — but aren’t familiar with their inner workings. Stork is intended to dispel some of the mystery around compilers, and its far enough along now to start discussing Stork in the greater context of compiler design.

In the most general sense, compilers are simply translators that turn program source code into executable instructions. There are many compilers: javac, the Java compiler, turns Java code into Java bytecode; gcc, the Gnu C Compiler, turns C code into native instructions, and so on. There are also similar programs called interpreters that execute program source code directly without first compiling them down to instructions, like ruby or the subject of this course, Stork. While interpreters are technically different from compilers, the same design principles apply, so the Stork interpreter will serve nicely as a platform for exploring simple compiler design.

Compiler Design: Front End, Middle End, and Back End

At a high level, compilers look like this:

Basic Compiler Design

Welcome back!

For those of you just joining, Stork is an example programming language “course” designed to demonstrate the principles of programming language implementation in 10 “lessons.” This is Lesson 2 in a series of 10, so if you’re just joining now, you may want to check out lesson 1 to gear up a bit for this story.

Lesson 2 covers the basics of parsing numerical expressions using the tokenizer implemented in Lesson 1. Evaluation of these expressions will be handled in Lesson 3.

The code for this lesson is available on github under the tag lesson2, and you can follow the discussion about this lesson on reddit.

What is Parsing?

If a programming language is a (more) convenient language humans can use to describe tasks to computers, then parsing is the process of turning a program’s tokens into sentences, or “abstract syntax trees” (ASTs), that the computer can understand. For example, consider this simple mathematical expression for the area of a circle with radius 5:

3.14159*5*5

For this program text, the tokens would be 3.14159, *, 5, *, 5, and a parser would build the following AST for it:

Clearly, parsing is essentially “sentence diagramming” for a programming language.

This lesson covers how to transform a token stream into a parse tree like the above example. Looking at parse trees — the syntactic relationships among tokens — instead of the tokens themselves will make evaluating those expressions much easier in the next lesson.

Welcome back!

For those of you just joining, Stork is an example programming language “course” designed to demonstrate the principles of programming language implementation in 10 “lessons.” This is Lesson 1 in the series, so if you’re just joining now, you haven’t missed much!

The code for this lesson is available on github under the tag lesson1, and you can follow the discussion about this lesson on reddit.

What is Tokenization?

If a programming language is a (more) convenient language humans can use to describe tasks and processes to computers, then tokenization is the process of turning a program’s raw program text into words, or “tokens,” that the computer can understand. For example, consider this simple Python program for the factorial function:

def factorial(n):
     if n == 0:
         return 1
     else:
         return n * factorial(n-1)

For this program text, the tokens would be: def, factorial, (, n, ), :, if, n, ==, and so on. Looking at tokens — atomic units of program semantics — as opposed to characters makes the next lesson’s topic of “parsing,” or discovering the semantic relationships among the different parts of the program text, much easier.

In a very real sense, the tokenizer defines the vocabulary of the programming language.

There are several questions every developer gets asked sooner or later:

• How do you learn to program?
• Should I learn to program?
• What is a program?
• Where do programs come from?
• What is a programming language?
• Where do programming languages come from?

Most of these questions already have excellent answers online. But while the question of where programming languages come from is the topic of some excellent books, I’ve never been able to find a simple, straightforward, free answer online.

I’m happy/sad this Michael Scott is not the author of that book.

Stork is a simple, free programming language I’m writing in ten steps and documenting in writing, code, and Reddit threads. I think Stork fills that hole, and I hope that you, gentle reader, think that it does, too.