examples: lexcalc: demonstrate location tracking

The bistromathic example should not use Flex, it makes it too complex.
But it was the only example to show location tracking with Flex.

* examples/c/lexcalc/lexcalc.test, examples/c/lexcalc/parse.y,
* examples/c/lexcalc/scan.l: Demonstrate location tracking as is done
in bistromathic.

examples/c/lexcalc/README.md

@@ -3,6 +3,8 @@
 This directory contains lexcalc, the traditional example of using Flex and
 Bison to build a simple calculator.
 
+It features detailed syntax errors with locations.
+
 <!---
 Local Variables:
 fill-column: 76

examples/c/lexcalc/lexcalc.test

@@ -29,4 +29,9 @@ run -noerr 0 9 -p
 cat >input <<EOF
 (1+2) *
 EOF
-run 1 'err: syntax error, unexpected end-of-line, expecting ( or number'
+run 1 'err: 1.8-2.0: syntax error, unexpected end-of-line, expecting ( or number'
+
+cat >input <<EOF
+1 / (2 - 2)
+EOF
+run 1 'err: 1.1-11: error: division by zero"

examples/c/lexcalc/parse.y

@@ -6,10 +6,10 @@
 {
   // Tell Flex the expected prototype of yylex.
 #define YY_DECL                                 \
-  enum yytokentype yylex (YYSTYPE* yylval, int *nerrs)
+  enum yytokentype yylex (YYSTYPE* yylval, YYLTYPE *yylloc, int *nerrs)
   YY_DECL;
 
-  void yyerror (int *nerrs, const char *msg);
+  void yyerror (YYLTYPE *loc, int *nerrs, const char *msg);
 }
 
 // Emitted on top of the implementation file.
@@ -33,6 +33,9 @@
 // Generate detailed error messages.
 %define parse.error detailed
 
+// with locations.
+%locations
+
 // Enable debug traces (see yydebug in main).
 %define parse.trace
 
@@ -78,7 +81,7 @@ exp:
   {
     if ($3 == 0)
       {
-        yyerror (nerrs, "invalid division by zero");
+        yyerror (&@$, nerrs, "error: division by zero");
         YYERROR;
       }
     else
@@ -90,9 +93,10 @@ exp:
 %%
 // Epilogue (C code).
 
-void yyerror (int *nerrs, const char *msg)
+void yyerror (YYLTYPE *loc, int *nerrs, const char *msg)
 {
-  fprintf (stderr, "%s\n", msg);
+  YY_LOCATION_PRINT (stderr, *loc);
+  fprintf (stderr, ": %s\n", msg);
   ++*nerrs;
 }
 

examples/c/lexcalc/scan.l

@@ -9,9 +9,24 @@
 #include <stdlib.h> /* strtol */
 
 #include "parse.h"
+
+  // Each time a rule is matched, advance the end cursor/position.
+#define YY_USER_ACTION                          \
+  yylloc->last_column += yyleng;
+
+  // Move the first position onto the last.
+#define LOCATION_STEP()                         \
+  do {                                          \
+    yylloc->first_line = yylloc->last_line;     \
+    yylloc->first_column = yylloc->last_column; \
+  } while (0)
 %}
 
 %%
+%{
+  // Each time yylex is called, move the head position to the end one.
+  LOCATION_STEP ();
+%}
  /* Rules.  */
 
 "+"      return TOK_PLUS;
@@ -27,17 +42,17 @@
   errno = 0;
   long n = strtol (yytext, NULL, 10);
   if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
-    yyerror (nerrs, "integer is out of range");
+    yyerror (yylloc, nerrs, "integer is out of range");
   yylval->TOK_NUM = (int) n;
   return TOK_NUM;
 }
 
+"\n"     yylloc->last_line++; yylloc->last_column = 1; return TOK_EOL;
+
  /* Ignore white spaces. */
-[ \t]+   continue;
+[ \t]+   LOCATION_STEP (); continue;
 
-"\n"     return TOK_EOL;
-
-.        yyerror (nerrs, "syntax error, invalid character");
+.        yyerror (yylloc, nerrs, "syntax error, invalid character"); continue;
 
 <<EOF>>  return TOK_EOF;
 %%