Drinkable Chicken

(Also see: part I, part II)

Just discovered something neat. In Python, if a function has arguments that have a default value, then those defaults are bound when the function is defined. So the following function, when called with no arguments, always returns the same value:

>>> def f(x=random.randrange(0, 100)): return x
...
>>> f()
32
>>> f()
32
>>> f()
32

...because the default value for x is determined when f is defined, rather than when it's called.

And this is not allowed at all:

>>> def z(a, b=a): print a, b
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined

I assumed that these rules would be the same in Chicken, but it turns out that this is not the case. This allows for some cool constructs that simply aren't possible in Python. Like the example with the random number:

> (define (f #!optional (x (random 100))) x)
> (f)
23
> (f)
89
> (f)
97

It appears that (random 100) is computed whenever f is called, rather than when it's defined. We can also refer to other arguments in this default expression:

> (define (g a #!optional (b (+ a 10)))
>   (list a b))
> (g 3)
(3 13)
> (g 40)
(40 50)

Good to know. This behavior is intentional rather than accidental, judging from the Extensions to the standard section in the user manual.