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From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: why learn C?
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Date: Thu, 05 Apr 2007 07:14:13 GMT
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Date: 2007-04-05T07:14:13+00:00
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jayessay wrote:
> C++ templates, other than being Turing complete, have absolutely nothing
 > to do with Lisp or Lisp like languages.  Nothing.  And to think otherwise
 > shows a complete ignorance of the matter.

Sorry, I'm afraid you're wrong about that. In fact, many C++ template
metaprograms use techniques that would be quite familiar to Lisp users.
For example, one might build up type lists as follows:

struct Nil { };
template <typename T1, typename T2 = Nil>
struct Cons { typedef T1 Car; typedef T2 Cdr; };

typedef Cons<int, Cons<bool, Cons<float> > > my3types;
my3types::Car an_int;
my3types::Cdr::Car a_bool;
my3types::Cdr::Cdr::Car a_float;

> Allow a user/programmer to define a dictionary like object while
> ensuring at _compile_ time that the size of the table implementing it
> is a prime number.
> 
> That's something that is easy to do in Lisp.  It is at least
> theoretically possible (if mind numbingly painful) to achieve with C++
> templates.

It's actually quite easy with templates, although the following may
hit compiler limits that a better algorithm would avoid. It takes
just three lines to implement straightforwardly the definition that
a prime number has exactly two divisors.

template<int N, int K> struct divNleK { enum { v = ((N % K) == 0) + divNleK<N,K-1>::v }; };
template<int N> struct divNleK<N, 1>  { enum { v = 1 }; };
template<int N> struct prime_test     { enum { v = divNleK<N,N>::v == 2 }; };

> That wouldn't be so bad (or even such a big deal), but C++ templates
> are nothing like this and are in general a clumsy giant PITA.

I don't agree at all.