Improve some documentation.

1 files changed, 20 insertions, 14 deletions

diff --git a/src/libsyntax/ext/tt/macro_rules.rs b/src/libsyntax/ext/tt/macro_rules.rs
index cd6ebce4394..a5fc20c461b 100644
--- a/src/libsyntax/ext/tt/macro_rules.rs
+++ b/src/libsyntax/ext/tt/macro_rules.rs
@@ -5,12 +5,10 @@ import ast::{ident, matcher_, matcher, match_tok,
 import parse::lexer::{new_tt_reader, tt_reader_as_reader, reader};
 import parse::token::{FAT_ARROW, SEMI, LBRACE, RBRACE, nt_matchers, nt_tt};
 import parse::parser::{parser, SOURCE_FILE};
-import earley_parser::{parse, success, failure, named_match,
+import earley_parser::{parse, parse_or_else, success, failure, named_match,
                        matched_seq, matched_nonterminal};
 import std::map::hashmap;
 
-
-
 fn add_new_extension(cx: ext_ctxt, sp: span, name: ident,
                      arg: ~[ast::token_tree]) -> base::mac_result {
     // these spans won't matter, anyways
@@ -18,6 +16,9 @@ fn add_new_extension(cx: ext_ctxt, sp: span, name: ident,
         {node: m, span: {lo: 0u, hi: 0u, expn_info: none}}
     }
 
+    // The grammar for macro_rules! is:
+    // $( $lhs:mtcs => $rhs:tt );+
+    // ...quasiquoting this would be nice.
     let argument_gram = ~[
         ms(match_seq(~[
             ms(match_nonterminal(@~"lhs",@~"matchers", 0u)),
@@ -25,43 +26,48 @@ fn add_new_extension(cx: ext_ctxt, sp: span, name: ident,
             ms(match_nonterminal(@~"rhs",@~"tt", 1u)),
         ], some(SEMI), false, 0u, 2u))];
 
+
+    // Parse the macro_rules! invocation (`none` is for no interpolations):
     let arg_reader = new_tt_reader(cx.parse_sess().span_diagnostic,
                                    cx.parse_sess().interner, none, arg);
-    let arguments = alt parse(cx.parse_sess(), cx.cfg(),
-                              arg_reader as reader, argument_gram) {
-      success(m) { m }
-      failure(sp, msg) { cx.span_fatal(sp, msg); }
-    };
+    let argument_map = parse_or_else(cx.parse_sess(), cx.cfg(),
+                                     arg_reader as reader, argument_gram);
 
-    let lhses = alt arguments.get(@~"lhs") {
+    // Extract the arguments:
+    let lhses:~[@named_match] = alt argument_map.get(@~"lhs") {
       @matched_seq(s, sp) { s }
       _ { cx.span_bug(sp, ~"wrong-structured lhs") }
     };
-    let rhses = alt arguments.get(@~"rhs") {
+    let rhses:~[@named_match] = alt argument_map.get(@~"rhs") {
       @matched_seq(s, sp) { s }
       _ { cx.span_bug(sp, ~"wrong-structured rhs") }
     };
 
+    // Given `lhses` and `rhses`, this is the new macro we create
     fn generic_extension(cx: ext_ctxt, sp: span, arg: ~[ast::token_tree],
                          lhses: ~[@named_match], rhses: ~[@named_match])
     -> mac_result {
+        // Which arm's failure should we report? (the one furthest along)
         let mut best_fail_spot = {lo: 0u, hi: 0u, expn_info: none};
         let mut best_fail_msg = ~"internal error: ran no matchers";
 
         let s_d = cx.parse_sess().span_diagnostic;
         let itr = cx.parse_sess().interner;
 
-        for lhses.eachi() |i, lhs| {
+        for lhses.eachi() |i, lhs| { // try each arm's matchers
             alt lhs {
               @matched_nonterminal(nt_matchers(mtcs)) {
+                // `none` is because we're not interpolating
                 let arg_rdr = new_tt_reader(s_d, itr, none, arg) as reader;
                 alt parse(cx.parse_sess(), cx.cfg(), arg_rdr, mtcs) {
-                  success(m) {
-                    let rhs = alt rhses[i] {
+                  success(named_matches) {
+                    let rhs = alt rhses[i] { // okay, what's your transcriber?
                       @matched_nonterminal(nt_tt(@tt)) { tt }
                       _ { cx.span_bug(sp, ~"bad thing in rhs") }
                     };
-                    let trncbr = new_tt_reader(s_d, itr, some(m), ~[rhs]);
+                    // rhs has holes ( `$id` and `$(...)` that need filled)
+                    let trncbr = new_tt_reader(s_d, itr, some(named_matches),
+                                               ~[rhs]);
                     let p = parser(cx.parse_sess(), cx.cfg(),
                                    trncbr as reader, SOURCE_FILE);
                     ret mr_expr(p.parse_expr());