Write Yourself a Scheme in 48 Hours

Overview

Most Haskell tutorials on the web use a style of teaching akin to language reference manuals. They show you the syntax of the language, a few language constructs, then tell you to create a few simple functions at the interactive prompt. The "hard stuff" of how to write a functioning, useful program is left to the end, or omitted entirely.

This tutorial takes a different approach. You'll start off using and parsing the command-line, then progress to writing a fully-functional Scheme interpreter that implements a decent subset of R⁵RS Scheme. Along the way, you'll learn Haskell's I/O, mutable state, dynamic typing, error handling, and parsing features. By the time you finish, you should become fairly fluent in Haskell and Scheme.

This tutorial targets two main audiences:

• People who already know Lisp or Scheme, and want to learn Haskell
• People who don't know any programming language, but have a strong quantitative background, and are familiar with computers

The second group will likely find this challenging, as this tutorial glosses over several concepts in Scheme and general programming in order to stay focused on Haskell. A good textbook such as Structure and Interpretation of Computer Programs or The Little Schemer should be very helpful.

Users of a procedural or object-oriented languages like C, Java, or Python should beware: You'll have to forget most of what you already know about programming. Haskell is completely different from those languages, and requires a different way of thinking about programming. It's best to go into this tutorial with a blank slate; try not to compare Haskell to imperative languages, because many concepts (including classes, functions, and return) have significantly different meanings in Haskell.

Since each lesson builds upon code written in previous ones, it's generally best to go through them in order.

This tutorial assumes that you'll use GHC as your Haskell compiler. The code may work with Hugs or other compilers, but hasn't been officially tested with them, and may require downloading additional libraries.

A prior, non-wiki-edited version of the source code files used in this tutorial is available on the original site.

Contents

1. First Steps: Compiling and running
2. Parsing
3. Evaluation, Part 1
4. Error Checking and Exceptions
5. Evaluation, Part 2
6. Building a REPL: Basic I/O
7. Adding Variables and Assignment: Mutable State in Haskell
8. Defining Scheme Functions: Closures and Environments
9. Creating IO Primitives: File I/O
10. Towards a Standard Library: Fold and Unfold
11. Conclusion & Further Resources
12. Answers to Exercises