Re: Accumulator

[Zooko <address@hidden>]

> takes a couple of classes from LWP (Perl's libwww) and the required
> control structures to spider and perform link checking, and implements
> something *more* functional and extensible in roughly half the code.

This reminds me of some Python code that I've written.  The function named 
`spider()' takes two arguments: an object in memory and a function.  It 
traverses all the transitively reachable objects in the Python runtime and 
invokes the function once on each.  Here's the code (also visible at [1]).

def spider(startnode, func, maxdepth=None):
    """
    visits each node once

    @param func a func which takes two arguments: the current object, the 
        previous object which refers to the current object; If the func 
        returns `-1', then spider() will *not* recurse on that object.  If the 
        func returns `-2', then spider() will exit.
    """
    # traverse (depth-first) all memory starting with `startlist' and for each object o, invoke `func(o, p)'
    a = {}
    return _rec_spider(startnode, a, func, maxdepth=maxdepth)

def _rec_spider(o, a, func, d=0, maxdepth=None):
    if o is a or o is locals():
        return  # skip things that we know may contain it or that we've already visited
    for nt in get_references(o):
        if not a.has_key(id(nt)):
            ret = func(nt, o)
            a[id(nt)] = None
            if ret == -2:
                return -2
            elif ret == -1:
                continue
            elif ((maxdepth is None) or (d < maxdepth)):
                if _rec_spider(nt, a, func, d=d+1, maxdepth=maxdepth) == -2:
                    return -2


This is 17 lines of code (excluding comments and doc strings), and the 
interface is flexible enough to do what I want.  Even if the Lisp version 
turns out to be so blindingly elegant and flexible that it is implemented in 
1 line of code and washes my dishes, I doubt that I'll care.  The code above 
is good enough for me, and I spend my day writing more complicated things 
(in Python), not rewriting this function to be ever more "elegant".

Regards,

Zooko

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[1] http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/pyutil/pyutil/memutil.py?rev=HEAD&content-type=text/vnd.viewcvs-markup