examples: clean up

* examples/c/calc/calc.y: Restore to its original state, with
parse.error=detailed instead of parse.error=custom (this example
should be simple).
* examples/c/calc/calc.test: Check syntax errors.
* examples/c/lexcalc/parse.y: Add comments.

examples/c/calc/calc.test

@@ -34,4 +34,10 @@ cat >input <<EOF
 (1+2) * 3
 EOF
 run 0 9
+# Check the traces.
 run -noerr 0 9 -p
+
+cat >input <<EOF
+1++2
+EOF
+run 0 "err: syntax error, unexpected '+', expecting number or '('"

examples/c/calc/calc.y

@@ -1,50 +1,35 @@
 %code top {
   #include <ctype.h>  /* isdigit. */
-  #include <stdbool.h>
   #include <stdio.h>  /* For printf, etc. */
   #include <string.h> /* strcmp. */
 
   int yylex (void);
   void yyerror (char const *);
-
-  bool show_expected = false;
-
-#define PRINT_EXPECTED_TOKENS()                                         \
-  do {                                                                  \
-    if (show_expected)                                                  \
-      {                                                                 \
-        yyparse_context_t ctx                                           \
-          = {yyssp, yytoken, yyesa, &yyes, &yyes_capacity};             \
-        int tokens[YYNTOKENS];                                          \
-        int cnt = yyexpected_tokens (&ctx, tokens, YYNTOKENS);          \
-        fprintf (stderr, "expected tokens in state %d rule %d (%d):",   \
-                 *yyssp, yyn - 1, cnt);                                 \
-        for (int i = 0; i < cnt; ++i)                                   \
-          fprintf (stderr, " %s", yysymbol_name(tokens[i]));            \
-        fprintf (stderr, "\n");                                         \
-      }                                                                 \
-  } while (0)
 }
 
 %define api.header.include {"calc.h"}
-%define api.value.type union /* Generate YYSTYPE from these types:  */
-%define parse.error custom
-%define parse.lac full
+
+/* Generate YYSTYPE from the types used in %token and %type.  */
+%define api.value.type union
 %token <double> NUM "number"
 %type  <double> expr term fact
 
-/* Generate the parser description file.  */
+/* Generate the parser description file (calc.output).  */
 %verbose
+
+/* Nice error messages with details. */
+%define parse.error detailed
+
 /* Enable run-time traces (yydebug).  */
 %define parse.trace
 
-/* Formatting semantic values.  */
+/* Formatting semantic values in debug traces.  */
 %printer { fprintf (yyo, "%g", $$); } <double>;
 
 %% /* The grammar follows.  */
 input:
-  %empty       { PRINT_EXPECTED_TOKENS (); }
-| input line   { PRINT_EXPECTED_TOKENS (); }
+  %empty
+| input line
 ;
 
 line:
@@ -66,34 +51,12 @@ term:
 ;
 
 fact:
-  "number"                                   { PRINT_EXPECTED_TOKENS (); }
-| '(' expr { PRINT_EXPECTED_TOKENS (); } ')' { $$ = $expr; }
+  "number"
+| '(' expr ')' { $$ = $2; }
 ;
 
 %%
 
-int
-yyreport_syntax_error (const yyparse_context_t *ctx)
-{
-  enum { YYERROR_VERBOSE_ARGS_MAXIMUM = 10 };
-  /* Arguments of yyformat: reported tokens (one for the "unexpected",
-     one per "expected"). */
-  int arg[YYERROR_VERBOSE_ARGS_MAXIMUM];
-  int n = yysyntax_error_arguments (ctx, arg, sizeof arg / sizeof *arg);
-  if (n == -2)
-    return 2;
-  fprintf (stderr, "SYNTAX ERROR on token [%s]", yysymbol_name (arg[0]));
-  if (1 < n)
-    {
-      fprintf (stderr, " (expected:");
-      for (int i = 1; i < n; ++i)
-        fprintf (stderr, " [%s]", yysymbol_name (arg[i]));
-      fprintf (stderr, ")");
-    }
-  fprintf (stderr, "\n");
-  return 0;
-}
-
 int
 yylex (void)
 {
@@ -130,9 +93,7 @@ main (int argc, char const* argv[])
 {
   /* Enable parse traces on option -p.  */
   for (int i = 1; i < argc; ++i)
-    if (!strcmp (argv[i], "-e"))
-      show_expected = 1;
-    else if (!strcmp (argv[i], "-p"))
+    if (!strcmp (argv[i], "-p"))
       yydebug = 1;
   return yyparse ();
 }

examples/c/lexcalc/parse.y

@@ -19,12 +19,24 @@
 #include <stdlib.h> // getenv.
 }
 
+// Don't share global variables between the scanner and the parser.
 %define api.pure full
+
+// To avoid name clashes (e.g., with C's EOF) prefix token definitions
+// with TOK_ (e.g., TOK_EOF).
 %define api.token.prefix {TOK_}
+
+// %token and %type use genuine types (e.g., "%token <int>").  Let
+// %bison define YYSTYPE as a union of all these types.
 %define api.value.type union
-%define parse.error verbose
+
+// Generate detailed error messages.
+%define parse.error detailed
+
+// Enable debug traces (see yydebug in main).
 %define parse.trace
- // Error count, exchanged between main, yyparse and yylex.
+
+// Error count, exchanged between main, yyparse and yylex.
 %param {int *nerrs}
 
 %token
@@ -77,6 +89,7 @@ exp:
 ;
 %%
 // Epilogue (C code).
+
 void yyerror (int *nerrs, const char *msg)
 {
   fprintf (stderr, "%s\n", msg);

examples/c/lexcalc/scan.l

@@ -1,4 +1,4 @@
-/* Prologue (directives).   -*- C++ -*- */
+/* Prologue (directives).   -*- C -*- */
 
 /* Disable Flex features we don't need, to avoid warnings. */
 %option nodefault noinput nounput noyywrap

examples/c/reccalc/scan.l

@@ -1,4 +1,4 @@
-/* Prologue (directives).   -*- C++ -*- */
+/* Prologue (directives).   -*- C -*- */
 
 /* Disable Flex features we don't need, to avoid warnings. */
 %option nodefault noinput nounput noyywrap
@@ -17,10 +17,14 @@
 
 %%
 %{
+  // Number of opened parentheses.
   int nesting = 0;
+  // A buffer storing the text inside the outer parentheses.
   char *str = NULL;
-  int size = 0;
+  // Its allocated size.
   int capacity = 0;
+  // Its used size.
+  int size = 0;
 #define STR_APPEND()                                    \
   do {                                                  \
     if (capacity < size + yyleng + 1)                   \