Perchance to Scheme

A field guide to the menagerie of Scheme dialects

Sunday, February 14, 2016 · 5 min read

Scheme, they say, is an idea rather than a language. But what does that mean? Is Java an idea or a language? C?

When you write code in Java or C, you expect it to work, more or less, even if you use a different compiler. gcc and clang are generally compatible. Similarly, you can run a Java program in Hotspot, JRockit, Kaffe, or J9 and it should be fine. You can compile the Java program with javac or gcj and it should work the same (one might be slower than the other, though).

This is because C and Java have specifications. There are long documents describing exactly what the languages should do, and someone writing a compiler needs to follow those specifications. The implementation details—how a feature works—is up to them, but not any of the actual language design.

Scheme, on the other hand, is very loosely specified. This is a good thing: the core language is so small that the entire standard fits on about 50 pages (Java’s specification fits on 644). Additional helpful features are included in “Scheme Requests for Implementation” or SRFIs, which are not a part of the standard but are useful for programmers—things like common list operations.

It’s not nebulous, it’s simply minimalist. This is why Scheme is more of an idea than a language.

As a result, implementing a Scheme compiler gives you a lot more freedom in how you want your language to look, while still calling it a “Scheme dialect”. You can choose to make square brackets legal delimiters, or you could choose not to. You almost always have to supply your own I/O primitives like “prompt for input” or “open a socket”. Module systems and importing are up to you.

And so Scheme programs are usually not compatible across implementations. With so many hundreds of Scheme dialects out there, it’s kind of overwhelming for a first-time Scheme programmer to pick a dialect and actually get started.

I’m a Scheme dialect nerd. I probably have more Scheme dialects installed than most people have games on their phone. Here are my opinions on which Scheme dialect to use. Rather than grouping by use-case, as guides such as this one do, I grouped by language.

Racket

Racket is, in a word, academic. It started off as PLT Scheme, which was essentially a research group that happened to produce a really good pedagogical Scheme dialect which they used for a lot of their research.

Racket has an impressive standard library and a decent module system. Its POSIX interface is a bit wanting, though. For example, you can’t send a signal to a process yet (technically, I have a PR open for this here). This is partly because Racket aims to be generally platform-independent, and so doesn’t necessarily want to implement a whole bunch of Unix-specific features in order to appeal to Windows users. This is either a good thing or a bad thing, depending on your use-case.

Racket’s FFI is iffy. Writing C extensions is kind of tricky.

There are a couple ways to distribute your Racket program: you can have users install Racket and download the source, or you can use raco to compile an executable that bundles Racket’s runtime system with compiled bytecode. Neither way seems perfect to me, but, well, they work.

Racket comes with a lot of frills. There’s a GUI engine, an IDE (which isn’t great, but isn’t awful either), and primitives for manipulating images and web servers and whatnot. It also has copious amounts of documentation, and it ships with its own documentation tool called Scribble (which is documented in Scribble). Racket is also a “language lab”: it gives you the tools to create your own programming languages built on top of Racket infrastructure (for example, Typed Racket, the type-safe dialect of Racket, is written in Racket).

Racket has a nice community and mailing list archives/StackOverflow answers for help. It’s also the recommended Scheme dialect to use if you’re learning or teaching Scheme/SICP.

If you’re still lost at the end of this article, stop thinking and go with Racket and you will be fine.

Chicken

Chicken is sort of the opposite of Racket. It’s small (R5RS only), but it has a fantastic POSIX API. Chicken implements the R5RS Scheme standard, which stands for the Revised Revised Revised Revised Revised Report on the Algorithmic Language Scheme.

Its website is call-cc.org because the Chicken devs are super-proud of how Chicken handles continuations: it uses the amazingly-named Cheney on the MTA strategy.

Continuations, by the way, are one of the coolest Scheme features.

Chicken has a decent number of libraries, and it’s really easy to turn a C module into a Chicken module.

Chicken compiles to C, so to distribute, you can either distribute binaries or C sources which the user can build without installing Chicken. Super-easy.

Clojure(Script)

ClojureScript is a compiler from Clojure to JavaScript (Clojure originally targeted Java). It is not a Scheme dialect, not does it feel like one. It’s a LISP dialect. Here are some differences between LISP and Scheme, if that interests you.

Personally, I don’t like it on a matter of principle: I don’t like things that compile to JavaScript. It doesn’t seem like anything you really need to write better code. They say JavaScript is Scheme in C’s clothing, anyway, and I do agree with that to some extent.

Same holds for BiwaScheme, Spock (written by the Chicken developer), the (unmaintained) WhaleSong Racket-to-JS compiler, and anything on this list.

Chez Scheme

The Chez Scheme compiler isn’t free, and its website lives on a COM domain. Take from that what you will.

That being said, it’s reputed to be ridiculously fast. I have never met a Chez Scheme user, but maybe I’m in the wrong crowd.

Guile Scheme

Guile is a GNU project, which means it has all sorts of ideals and mission statements and declarations that make it seem like the paragon of freedom.

In short, Guile is the Scheme you want to use in “embedded” form. That is, you can stick a Guile interpreter in a C application in order to control it without having to deal with C. One of the canonical examples of this is the turtle graphics example, in which you write a graphics display in C, and then let the user control it with Scheme, which is easier. Similarly, the WeeChat IRC chat client lets you write extensions in Guile (for example, an extension that automatically URL-shortens long URLs before you send them).

Guile can both interpret and compile Scheme, it has good documentation, and it’s backed by GNU so you can expect maintained code (Guile is used by other mature GNU projects such as gdb , so they’re invested). Also, Guile is written in Guile, and the LilyPond music formatting program is written in Guile.

Emacs Lisp

You only use Emacs Lisp if you, well, use Emacs, just like how you only use VimScript if you use Vim.

Anything else

Before choosing any other Scheme implementation, it’s probably worth your time to make sure it really does offer something more than the dialects I listed above. Make sure it has an active community that can help you when you run into problems, and make sure you can find code written by other people in that dialect of Scheme. A good quality test is to see if it implements any SRFIs.

Then, hack away.

And now, a brief word on the proliferation of Scheme dialects.

Part of the problem is that one you pick a Scheme implementation, you’re almost always “locked in”: you can’t easily migrate to another. So people tend to just build their own Scheme environment that they can control.

Another part is that every CS student and their pet dog has probably written some approximation of a Scheme interpreter (possibly as a course project), so the Babel effect kicks in and creates a plethora of implementations of choose from. I’m guilty of this.

It’s just so easy to write a Scheme implementation. In a way, Scheme is a virus… but that’s the subject of a future post (or you could just read Snow Crash by Neal Stephenson and stick “programming” before every instance of “language”).

An upside of this, though, is that as the Scheme virus infects the programming language community, it injects the Functional Programming DNA everywhere. Once you’re immune to Scheme (by reading SICP), you’ll thrive in any such environment, be it JavaScript, Scala, Haskell, or Ruby.

Oh, and one last thing. I feel it’s obligatory at this point for me to say, please don’t spend too much time researching Scheme dialects. Just pick Racket and start coding.