Drinkable Chicken

Python has a useful idiom, that allows one to use the same file as both a module and a program. Consider this simple example:

# foo.py

def bar(x):
    print "bar says:", x

if __name__ == "__main__":

    bar(42)

The if __name__ == “__main__” clause is only executed if foo.py is run as the “main program”. In other words, we can do both

$ python foo.py
bar says: 42

and

$ python
Python 2.5.1 (r251:54869, Apr 18 2007, 22:08:04)
[GCC 4.0.1 (Apple Computer, Inc. build 5367)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import foo
>>> foo.bar(33)
bar says: 33

Although admittedly a bit of a hack, this construct is well-known and often used.

Now, on to Chicken Scheme. Can we do the same? It required a bit of poking around in documentation and mailing list, but it appears the answer is yes.

;; foo.scm

(define (foo x)
  (print "foo says: " x))

(define (main args)
  (print "args: " args)
  (foo 42))

Now, we can run this as a script with csi -ss (which looks for a function called main and automagically calls it):

$ csi -ss foo.scm
args: ()
foo says: 42

Notice that the main function has an argument args, which contains the command line arguments passed to the program:

$ csi -ss foo.scm 1 2 3
args: (1 2 3)
foo says: 42

We can also import foo.scm from within an interactive session, in which cases main is not called:

$ csi

CHICKEN
Version 3.0.0 - macosx-unix-gnu-x86     [ manyargs dload ptables applyhook ]
(c)2000-2008 Felix L. Winkelmann        compiled 2008-03-05 on niflheim.local (Darwin)

; loading /Users/zephyrfalcon/.csirc ...
; loading /usr/local/lib/chicken/3/readline.so ...
#;1> (use foo)
; loading ./foo.scm ...
#;2> (foo 101)
foo says: 101

But, but! Isn’t Chicken primarily a compiler? Does the above work too when using csc rather than csi? Actually it does, but the invocation is different. I use the following:

$ csc foo.scm -postlude "(main (cdr (argv)))"

$ ./foo
args: ()
foo says: 42

$ ./foo 1 2 3
args: (1 2 3)
foo says: 42

The -postlude option can be used to specify code that runs when the executable is called. (Actually, the official explanation is: “Add EXPRESSIONS after all other toplevel expressions in the compiled file. This option may be given multiple times. Processing of this option takes place after processing of -epilogue.”)

I use (main (cdr (argv))) as the postlude expression, which seems to pass command line arguments the same way as csi -ss passes them to the main function (although there might be a catch that I’m not aware of). The cdr is necessary because the first item of the list returned by (argv) is the name of the calling program (e.g. “./foo”).

(If there’s a better way, please let me know, as my knowledge about the compiler is limited.)

Next up: parsing command line arguments in both Python and Scheme…