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From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.lang.ada
Subject: Re: ADA Popularity Discussion Request
Date: Thu, 9 Sep 2004 03:01:59 +0000 (UTC)
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Date: 2004-09-09T03:01:59+00:00
List-Id: <comp.lang.ada>

Kevin Cline <kevin.cline@gmail.com> wrote:
: If you want bounded types in C++ it's easy enough to write a template
: class:

True, but it is not quite the same thing, is it? You can write bounded
types in C. Yet, they don't become C language types, with ISO definded
semantics beyond what you'd expect for any user defined type.
Does the C++ standard directly address the behavior of handwritten
bounded numbers? How many lines of template code will it take
to get the effect of

  type A is array(Some_Index_Type) of Foo;?


#include <assert.h>

struct _bint {
  long v;
  int up, lo;
};
typedef struct _bint bint;  // int with bounds checking

static inline void throw_C_overflow() { assert(0); }

bint range_checked(bint val) {
  if (val.lo > val.v || val.v > val.up) throw_C_overflow();
  return val;
}

bint make_bound(int lower, int upper, int val) {
  return range_checked((bint){ .v = val, .up = upper, .lo = lower });
}
bint multiply(bint a, bint b, bint res) {
  return range_checked((bint){ .v = a.v * b.v, .lo = res.lo, .up = res.up });
}
void set(bint* old, int new) {
  (*old).v = new;
  range_checked(*old);
}

int main()
{
  bint x = make_bound(1, 10, 3);

  // set(&x, 14);   // try this to see the effect
  return multiply(x, x, x).v;
}

: In LISP-y languages, you would just write something like:
:  (defun random-index () (bound 1 10 ... )

OTOneH people complain that large C programs mimic half of Lisp.
 (C being a compiled language with little support for anything.)
OTOtherH people suggest that Ada features be implemented in Lisp.
 (Ada being just a compiled language with a few nice features.)
Adjust for C++ and other combinations.
Now what?

-- Georg