If you’re coming to Racket from another REPL language (such as another Lisp), this post might be real Captain Obvious material.
But if you’re coming to Racket from an edit/compile/debug language like C or C++, it might be unclear what a typical workflow is. You might have questions like:
- How do I compile?
- How do I debug?
After seeing a question like this recently1, I figured I’d write about my typical workflow in DrRacket or Emacs. It is:
- Write a small function, in a source file.
- Try calling the function, in the REPL.
- Write some more examples of calling it, in the source file.
- If any problems, fix them and go to 2.
- Change the examples into
Let’s walk through these steps with a simple example.
Write a small function
Try calling it in the REPL
What is “the REPL”? REPL stands for Read Eval Print Loop. You have a prompt, and can enter Racket expressions to be evaluated.
DrRacket calls this the “interactions” pane, on the right or bottom.
Emacs racket-mode calls this the
In both cases:
The REPL appears the first time you choose Run, for example by pressing F5.
The REPL will be “inside” the file (module) that you’re editing — meaning that things defined in the file are visible and usuable from the REPL.
In the REPL, try calling
twice with various values and see what happens:
1 2 3 4
file.rkt> (twice 0) 0 file.rkt> (twice 2) 4
Put the examples in the source file
Some of these examples you try may be interesting. Copy them from the REPL to the source file:
When you run the file, you’ll see this output in the REPL:
Expressions at the top level in a file — such as these function applications — print their values when the file is run.
There’s nothing to fix in this example.
twice is behaving as we expect. But if you make fixes, it’s easy to quickly try the revised version, as we saw above.
Change the examples into unit tests
Next, simply nest your examples inside:
1 2 3 4 5 6
So our example becomes:
1 2 3 4 5 6 7 8 9
If I’m sure the function result value is correct, I’ll often just copy the output from the REPL into the right hand side of the
But only after checking carefully that it’s correct. Nothing sucks like a unit test whose expected value came from a buggy actual output. (Not that I would make that mistake, but I have a friend who knows someone who did this once. *coughs*)
And that’s it. This code example is extremely simple, but even with more realistic examples, this is my usual workflow:
- Make simple functions.
- Try them “live” in the REPL.
- Accumulate some interesting examples.
- When things look good, “bake” the examples as unit tests.
What about debugging?
In C/C++, I was from the school that, whenever you write new code, the very first thing you do is step through it in the debugger. You don’t just hit “Go” and look at the results. Instead, you step through, line by line, and see that it’s behaving the way you want. I believe that’s a worthwhile discipline in a language where every line of your program is mutating a variable.
When I first learned Racket, I figured it would be similar. But as I showed above, that’s not really the case. Although DrRacket has a GUI debugger, and it works, I rarely use it. Instead, the interactive REPL serves much of the same purpose — especially when you’re writing small functions that don’t mutate state. You can interact with those functions, and see how they actually behave with various inputs.
Mostly that’s all I need. But when that’s not enough, a good old-fashioned
printf usually suffices. It may sound too simple, but when I ask very experienced Racket developers, that’s what they do.
Now, if you find yourself sticking
printfs at the start of more than a couple functinos to see the call path — there’s an easier way:
(require racket/trace), then put
(trace function-name) in your source or in the REPL to include a function in the trace, or
function-name) to remove it.
Finally, when it comes to debugging macros, DrRacket’s macro stepper is quite brilliant. Not only does it show the expansion steps (which you can also get using
expand at the REPL), it draws errors to show the origin of identifiers and lets you inspect the full syntax objects. You may not need it for a simple macro, but when you need it, it’s a lifesaver.
I lie. I wrote most of this post 8 months ago. It’s been sitting around as a draft. I finally decided to review and publish it. ↩