Make a language in one hour: stacker

The compo­nents of a language

Every language made with Racket has two essen­tial compo­nents:

For every language, we’ll specify a reader and an expander. Some­times we’ll write them from scratch. Some­times we’ll import them from else­where. But they’re always manda­tory.

They don’t need to be large, either. For stacker, we’ll write both the reader and expander. But they’ll both fit into 40 lines of code.

Boot­strap­ping our language

Every Racket source file begins with a #lang line. If you’ve explored Racket you might have seen vari­ants like these:

In this book, we’ll primarily be using:

As we just learned, the first step in inter­preting a language is to send the source code to the reader for that language. The #lang line’s job is to tell Racket where to find that reader. The reader, in turn, will tell Racket where to find the expander.

The reader and expander for a language are often stored in different source files—or modules, as they’re also known in Racket. But that’s optional. For this project, we’ll imple­ment the language with just one source file, called "stacker.rkt".

In the future, we’ll prefer to use the short­hand #lang name nota­tion shown in the above exam­ples. But that requires some extra setup. (We’ll learn how to do that in the next tuto­rial.)

For now, we’ll use the alter­nate #lang reader path nota­tion, where path is a local path to the source file that contains our reader:

Racket basics

We need to write some Racket code. So let’s learn some Racket:

Beyond that, Racket has the usual numbers, strings, and func­tions that we know from other languages. We’ll handle those as we encounter them.

Setting up our source files

Open DrRacket. Start a new source file called "stacker.rkt" and save it in a conve­nient loca­tion. We’ll make this file with a special tuto­rial language called br/quicklang. The br/quicklang language comes from the beautiful-racket package we installed during the main setup. It works the same way as ordi­nary Racket, but adds some extra conve­niences. For testing purposes, let’s add a second line, so our whole file looks like this:

Click Run in the DrRacket toolbar—or learn the key shortcut, because we’ll be using it a lot. In the DrRacket inter­ac­tions window, we’ll see the result:

In the same direc­tory as "stacker.rkt", create a second source file called "stacker-test.rkt". We’ll use this to test our new language as we go (but it won’t work yet):

If we run this file anyway, we get an error that looks like this:

If so, that’s the right result: we’re telling #lang to get a reader out of "stacker.rkt", but we haven’t made one yet.

So let’s do that.