dynamic classes and duck typing

[dsimcha]

== Quote from BCS (none at anon.com)'s article
> Show me ONE thing that can be done using run time meta programming that can't
> be done as well or better with run time, non-dynamic, non-meta and/or compile
> time meta. Unless I'm totally clueless as to what people are talking about
> when they say runtime meta, I don't think you will be able to. Anything that
> amounts to making the syntax look nicer can be done as compile time meta
> and anything else can be done with data structure walking and interpretation.
> All of that is available in non dynamic languages.
> I guess I should concede the eval function but if you don't like CTFE+mixin...

Oh come on.  I'm as much a fan of D metaprogramming as anyone, but even I admit
that there are certain things that static languages just suck at.  One day I got
really addicted to std.algorithm and decided I wanted similar functionality for
text filters from a command line, so I wrote map, filter and count scripts that
take predicates specified at the command line.

filter.py:

import sys

pred = eval('lambda line: ' + sys.argv[2])
for line in open(sys.argv[1]):
    if pred(line) :
        print line.strip()

Usage:
filter.py foo.txt "float( line.split()[1]) < 5.0"


Metaprogramming isn't very rigorously defined, but this has to qualify.  Try
writing something similar in D.