Result of an unsuccessful matching is label + subject

4 files changed, 19 insertions, 7 deletions

diff --git a/examples/listId1.lua b/examples/listId1.lua
index e075bd1..0bf26a1 100644
--- a/examples/listId1.lua
+++ b/examples/listId1.lua
@@ -8,12 +8,12 @@ local g = m.P{
 }
 
 function mymatch (g, s)
-        local r, e = g:match(s)
+        local r, e, sfail = g:match(s)
         if not r then
                 if e == 1 then
-        return r, "Error: expecting an identifier"
+        return r, "Error: expecting an identifier before '" .. sfail .. "'"
         elseif e == 2 then
-        return r, "Error: expecting ','"
+        return r, "Error: expecting ',' before '" .. sfail .. "'"
         else
         return r, "Error"
         end
diff --git a/lptree.c b/lptree.c
index cbdd4df..ee76af7 100644
--- a/lptree.c
+++ b/lptree.c
@@ -1198,10 +1198,12 @@ static int lp_match (lua_State *L) {
   size_t i = initposition(L, l);
   int ptop = lua_gettop(L);
   Labelset labelf; /* labeled failure */
+  const char *sfail = NULL; /* labeled failure */
   lua_pushnil(L);  /* initialize subscache */
   lua_pushlightuserdata(L, capture);  /* initialize caplistidx */
   lua_getfenv(L, 1);  /* initialize penvidx */
-  r = match(L, s, s + i, s + l, code, capture, ptop, &labelf); /* labeled failure */
+  r = match(L, s, s + i, s + l, code, capture, ptop, &labelf, &sfail); /* labeled failure */
+        /*printf("sfail = %s\n", sfail);*/
   if (r == NULL) { /* labeled failure begin */
     int j = 0;
     int n = 1;
@@ -1210,10 +1212,12 @@ static int lp_match (lua_State *L) {
       if (labelf & (1 << j)) {  
         lua_pushinteger(L, j);
         n++;
+                                break; /* Changing the semantics: only one label */
       }
       j++;
     }
-    return n;
+                lua_pushstring(L, sfail); /* Pushing the subject where the failure occurred */
+    return n + 1;
   }  /* labeled failure end */
   return getcaptures(L, s, r, ptop);
 }
diff --git a/lpvm.c b/lpvm.c
index a9198ba..3ae22f5 100644
--- a/lpvm.c
+++ b/lpvm.c
@@ -146,7 +146,7 @@ static int removedyncap (lua_State *L, Capture *capture,
 ** Opcode interpreter
 */
 const char *match (lua_State *L, const char *o, const char *s, const char *e,
-                   Instruction *op, Capture *capture, int ptop, Labelset *labelf) { /* labeled failure */
+                   Instruction *op, Capture *capture, int ptop, Labelset *labelf, const char **sfail) { /* labeled failure */
   Stack stackbase[INITBACK];
   Stack *stacklimit = stackbase + INITBACK;
   Stack *stack = stackbase;  /* point to first empty slot in stack */
@@ -184,6 +184,7 @@ const char *match (lua_State *L, const char *o, const char *s, const char *e,
         if (s < e) { p++; s++; }
         else {
           *labelf = LFAIL; /* labeled failure */
+                                        *sfail = s;
           goto fail;
         }
         continue;
@@ -197,6 +198,7 @@ const char *match (lua_State *L, const char *o, const char *s, const char *e,
         if ((byte)*s == p->i.aux && s < e) { p++; s++; }
         else {
           *labelf = LFAIL; /* labeled failure */
+                                        *sfail = s;
           goto fail;
         }
         continue;
@@ -212,6 +214,7 @@ const char *match (lua_State *L, const char *o, const char *s, const char *e,
           { p += CHARSETINSTSIZE; s++; }
         else {
           *labelf = LFAIL; /* labeled failure */
+                                        *sfail = s;
           goto fail;
         }
         continue;
@@ -227,6 +230,7 @@ const char *match (lua_State *L, const char *o, const char *s, const char *e,
         int n = p->i.aux;
         if (n > s - o) {
           *labelf = LFAIL; /* labeled failure */
+                                        *sfail = s;
           goto fail;
         }
         s -= n; p++;
@@ -297,6 +301,8 @@ const char *match (lua_State *L, const char *o, const char *s, const char *e,
       }
       case IThrow: { /* labeled failure */
         *labelf = (p+1)->labels;
+                                *sfail = s;
+                                /*printf("s = %s, sfail = %s\n", s, *sfail);*/
         goto fail;
       }
       case IFailTwice:
@@ -305,6 +311,7 @@ const char *match (lua_State *L, const char *o, const char *s, const char *e,
         /* go through */
       case IFail:
       *labelf = LFAIL; /* labeled failure */
+                        *sfail = s;
       fail: { /* pattern failed: try to backtrack */
         do {  /* remove pending calls */
           assert(stack > getstackbase(L, ptop));
@@ -327,6 +334,7 @@ const char *match (lua_State *L, const char *o, const char *s, const char *e,
         res = resdyncaptures(L, fr, s - o, e - o);  /* get result */
         if (res == -1) { /* fail? */
           *labelf = LFAIL; /* labeled failure */
+                                        *sfail = (const char *) s; /* TODO: ??? */
           goto fail;
         }
         s = o + res;  /* else update current position */
diff --git a/lpvm.h b/lpvm.h
index c996e22..cec9f5a 100644
--- a/lpvm.h
+++ b/lpvm.h
@@ -54,7 +54,7 @@ typedef union Instruction {
 
 void printpatt (Instruction *p, int n);
 const char *match (lua_State *L, const char *o, const char *s, const char *e,
-                   Instruction *op, Capture *capture, int ptop, Labelset *labelf); /* labeled failure */
+                   Instruction *op, Capture *capture, int ptop, Labelset *labelf, const char **sfail); /* labeled failure */
 
 
 #endif