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Macros in C3 - a status update

lerno profile image Christoffer Lernö ・3 min read

I'm going to share a bit of the C3 design process here for people who might be interested.

Like error handling, macros are one of the few truly new things in C3 compared to C. Consequently I've been going back and forth with the design trying to cover all angles.

I always wanted to make macros sufficiently safe that people could use them without worries, which means that some macro use from C would have to go, but which one?

After doing an inventory of what macros could do, I roughly end up with this "feature ladder" for macros – from easily understandable and readable to more "dangerous" in terms of how easy it would be to abuse:

  1. Inlining
  2. Lazy evaluation of arguments
  3. Polymorphic parameters
  4. Non-local jumps
  5. Implicit capture
  6. Declarations escaping scope
  7. Arbitrary code generation
  8. Code fragment replacement

One has to make the cut somewhere, and for C3 I think it's reasonable to either stop at (4) or (5).

(5) - implicit capture - is a bit related to (8) but can often be extremely useful in local code.

One hard-to-place feature is taking a name or a function invocation and then generating statements from that.

Consider the following:

#define FOO(X) do { X(0); X(1); X(2); } while 0;
void doX(int i) { ... }
FOO(doX);

In C3 this is sort of covered at the (2) level, even though for C that would be (7).

Because macros are mainstream tools in C3 rather than advanced tools it’s important that the syntax is geared towards writing code for 1-3 in particular.

This naturally makes it more natural to require that the macros should resemble functions as much as possible.

(6), (7) and (8) are, when used, usually clever ways to twist C into being more brief or to have an in-code DSL.

This flexibility can create pretty neat hacks, but it’s unclear whether this is a good idea in the large. Are these just clever solutions or are they important ones? My bet is on the former: that the legitimate uses more are about closing holes in C. And if it is, then the macros are basically a poor man's syntax extensions.

If syntax extensions are desired, Kit shows how that can be done in a very elegant manner. However, syntax extensions will always sacrifice readability for power, and here C3 makes a different tradeoff so that no matter what macro you see, you should be able to make a good guess as to what it could be doing.

For comparison, here are some C macros and their counterpart in C3 (as the design currently stands):

C:

#define nodesGet(nodes, index) ((INode**)((nodes)+1))[index]

C3:

macro INode *nodesGet(nodes, index)
{
  return cast(node + 1, INode**)[index];
}

C3 allows trailing body in macros, which makes for slightly different look from C in "foreach" style macros:

#define namespaceFor(ns) for (size_t __i = 0; __i < (ns)->avail; ++__i)

namespaceFor(ns) {
  NameNode *nn = &ns->namenodes[__i];
  if (nn->name == NULL)
    continue;
  nametblHookNode(nn->name, nn->node);
}

C3 (note that the declaration for trailing body is very much undecided):

macro namespaceFor(ns; void(usize i) $body)
{
  for (usize index = 0; index < ns.avail; index++)
  {
    body(index);
  }
}

@namespaceFor(ns; usize i) 
{
  NameNode *nn = &ns->namenodes[i];
  if (nn->name == NULL) continue;
  nametblHookNode(nn->name, nn->node);
}

Using implicit capture of variables from the surrounding scope:

#define lexReturnPuncTok(tok, skip) { \
  lex->toktype = tok; \
  lex->tokp = srcp; \
  lex->srcp = srcp + (skip); \
  return; \
}

C3:

macro lexReturnPuncTok!(tok, skip, implicit lex, implicit srcp)
{
  lex.toktype = tok;
  lex.tokp = srcp;
  lex.srcp = srcp + skip;
  return;
}

Creating a good macro system that is simple enough not to be dangerous requires difficult trade offs, and it's easy to just make it as flexible as possible. That might be a mistake though, with macros becoming an advanced feature reserved for special situations instead of a regular tool in the toolkit.

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