Jim Blandy wrote:

> In Pika:
> - Forgetting an UNGCPRO corrupts the GC's data structures, and may
>   fail only intermittently.  Irregular exits (return; goto; break;
>   continue) require attention.  longjmp is even harder.
> - Functions may only return Scheme values by reference; they may not
>   provide them as their (syntactic) return values.  Instead of writing
>   "f (g (x))", you must write:
>
>     g (&frame.x, &frame.temp);
>     f (&frame.temp, &frame.temp2);
>
>   In other words, you must write your code as linear series of
>   operations which work by side-effects.
> - Since the API functions all expect pointers to t_scm_word values,
>   this discourages people from passing them around directly, but it
>   can still be done --- e.g. "frame.x = frame.y;" --- and doing so
>   will usually work.  But doing so is a bug.
> - Variable declarations are cluttered with enclosing structs and GCPRO
>   / UNGCPRO calls.

This of course cries out for a C++ interface which can alleviate
these problems:

class ScmVal
{
   void *ref;
   ~ScmVal() { UNGCPRO(ref); }
   ... copy constructor etc ...;
}

Then f(g(x)) becomes:
f(g(x))
assuming:
ScmVal g (ScmVal x) { ... }
--
	--Per Bothner
xxxxxx@bothner.com   http://per.bothner.com/