Debugging layers

> As the compiler creates an AST from the parse tree, it copies src infos over (sometimes from tokens, sometimes from non-terminals).

In the TXR language, the same thing is done by the function rlcp (and a few relatives). I don't remember what "rl" stands for: probably read-time location.

You can see rlcp and rl calls throughout this parser:

http://www.kylheku.com/cgit/txr/tree/parser.y

In the evaluation module, http://www.kylheku.com/cgit/txr/tree/eval.c you can see this code:

    } else if ((macro = lookup_mac(menv, sym))) {
      val mac_expand = expand_macro(form, macro, menv);
      if (mac_expand == form)
        return form;
      form = rlcp_tree(mac_expand, form);
      goto tail;

User-defined macros aren't expected to do rlcp diligently; so we compensate for their negligence with rlcp_tree(mac_expand, form). This basically liberally sprinkles the source location of the original form over the nodes of the macro-expansion. It works pretty well in practice, helping to relate errors in macro expansions to the original source. rlcp_tree respects nodes that already have source info; it gives it to those which don't have it.

Source info is associated with tree nodes using a weak global hash table. When an object with source info attached goes away, the info goes away.

Trivial test:

macro-error-test.tl:

   1: (defmacro mac (expr)
   2:   expr)
   3: (mac (/ 1 0))

   $ txr macro-error-test.tl 
   txr: unhandled exception of type numeric-error:
   txr: possibly triggered at macro-error-test.tl:3 by form (/ 1 0)
   txr: message: /: division by zero

If we change the example to this:

   1: (defmacro mac (expr)
   2:   (copy expr))
   3: (mac (/ 1 0))

the error is exactly the same. This time the expr coming out of the macro is a copy and so it has no source info. The macro expander adds it. This is less accurate: the line number of the macro call isn't necessarily that of the (/ 1 0). Here it happens to be because it's a one-liner expression.

Example of an error during macro expansion:

   1: (defmacro mac (expr)
   2:   (/ 1 0))
   3: (mac foo)

   $ txr macro-error-test-2.tl 
   txr: unhandled exception of type numeric-error:
   txr: possibly triggered at macro-error-test-2.tl:2 by form (/ 1 0)
   txr: during expansion at macro-error-test-2.tl:3 of form (mac (/ 1 0))
   txr: message: /: division by zero