Series: racket February 24, 2014
Updated on Sat, 01 Mar 2014 23:37:57 +0700: Thanks to
Takikawa-san (asumu on IRC) for pointing out
the use of
main submodule. Now all you have to do is to declare a
#lang racket (module+ main (displayln "¡Hola mundo!"))
And you can run
racket command without any argument to achieve the same
This is the question I’ve encountered several times helping people learning Racket. The default behavior is a little unexpected, so I think I’d note it here as a short entry for whom it may be of help.
If you’re coming from the Python world, you’d probably write a Python code as followed:
#!/usr/bin/env python # Some code # ... if __name__ == "__main__": run_this_module_as_script()
Meaning your Python module could be executed as script.
run_this_module_as_script() is called only when this module is treated as
The same technique could be performed by Rubyists though it’s highly discouraged since the standard Ruby design pattern is to separate modules and scripts:
#!/usr/bin/env ruby # Some code # ... if __FILE__ == $0 run_this_module_as_script end
Let’s get back to Racket. Suppose we have this simple Racket module:
#lang racket (provide main) (define (main) (displayln "¡Hola mundo!"))
To achieve the same effect, reading
racket --help, one might be tempted to use:
racket -m a-module.rkt
to run the
main function, which (unexpectedly) fails.
Inexperienced Racket programmers
might tell you to remove the
#lang line and change it to a
works but your file is no longer a module, which most of the time is a
horrible idea. If you don’t know what it means, go read
Unless you know exactly what you’re doing, do NOT write non-module Racket.
Reading carefully, you’d see that the
Racket guide shows that for
Racket to process other flags, you need to explicitly tell Racket to do so
racket -tm a-module.rkt
Personally, I think it’s a bad design decision as it surprises new users. Still, that’s how it’s supposed to work for a long time. Behaviors from the Racket world don’t follow other popular languages. Sometimes they’re better, and sometimes they’re worse. Watch out for these moments and always read the documentation carefully.