dynamic classes and duck typing

[Andrei Alexandrescu]

dsimcha wrote:
> == 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.

eval rocks.

Andrei