diff --git a/src/hammer.h b/src/hammer.h
index ae2103ef7efcd8b321b39f2aa9778f39455f9ef9..25141e081f8502ead70c6ff797157cf3cf1cafa6 100644
--- a/src/hammer.h
+++ b/src/hammer.h
@@ -462,6 +462,15 @@ HAMMER_FN_DECL_NOARG(HParser*, h_nothing_p);
*/
HAMMER_FN_DECL_VARARGS_ATTR(H_GCC_ATTRIBUTE((sentinel)), HParser*, h_sequence, HParser* p);
+/**
+ * Given an `h_sequence` and a list of indices, returns a parser that parses the sequence
+ * but returns it without the results at the dropped indices. If a negative integer appears
+ * in the middle of the list, this combinator will silently ignore the rest of the list.
+ *
+ * Result token type: TT_SEQUENCE
+ */
+#define h_drop_from(p, ...) h_drop_from_(p, __VA_ARGS__, -1)
+HAMMER_FN_DECL_VARARGS(HParser*, h_drop_from_, HParser* p);
/**
* Given an array of parsers, p_array, apply each parser in order. The
* first parser to succeed is the result; if no parsers succeed, the
diff --git a/src/parsers/sequence.c b/src/parsers/sequence.c
index 5ca034ad50c1b752ff48e202008b21c0d1e75375..786ba62e43683f32ca0cc244bc0695cdb04a76fd 100644
--- a/src/parsers/sequence.c
+++ b/src/parsers/sequence.c
@@ -174,3 +174,85 @@ HParser* h_sequence__ma(HAllocator* mm__, void *args[]) {
ret->desugared = NULL;
return ret;
}
+
+HParser* h_drop_from_(HParser* p, ...) {
+ assert_message(p->vtable == &sequence_vt, "drop_from requires a sequence parser");
+ va_list ap;
+ va_start(ap, p);
+ HParser* ret = h_drop_from___mv(&system_allocator, p, ap);
+ va_end(ap);
+ return ret;
+}
+
+HParser* h_drop_from___m(HAllocator* mm__, HParser* p, ...) {
+ assert_message(p->vtable == &sequence_vt, "drop_from requires a sequence parser");
+ va_list ap;
+ va_start(ap, p);
+ HParser* ret = h_drop_from___mv(mm__, p, ap);
+ va_end(ap);
+ return ret;
+}
+
+HParser* h_drop_from___v(HParser* p, va_list ap) {
+ assert_message(p->vtable == &sequence_vt, "drop_from requires a sequence parser");
+ return h_drop_from___mv(&system_allocator, p, ap);
+}
+
+HParser* h_drop_from___mv(HAllocator* mm__, HParser *p, va_list ap) {
+ /* Ok, here's where things get funny.
+ *
+ * Saying `h_drop_from(h_sequence(a, b, c, d, e, NULL), 0, 4, -1)` is functionally
+ * equivalent to `h_sequence(h_ignore(a), b, c, d, h_ignore(e), NULL)`. Thus, this
+ * term rewrites itself, becoming an h_sequence where some parsers are ignored.
+ */
+ HSequence *s = (HSequence*)(p->env);
+ size_t indices[s->len];
+ size_t count = 0;
+ int arg = 0;
+
+ for (arg = va_arg(ap, int); arg >= 0; arg = va_arg(ap, int)) {
+ indices[count] = arg;
+ count++;
+ }
+ va_end(ap);
+
+ HSequence *rewrite = h_new(HSequence, 1);
+ rewrite->p_array = h_new(HParser *, s->len);
+ rewrite->len = s->len;
+ for (size_t i=0, j=0; i<s->len; ++i) {
+ if (indices[j]==i) {
+ rewrite->p_array[i] = h_ignore(s->p_array[i]);
+ ++j;
+ } else {
+ rewrite->p_array[i] = s->p_array[i];
+ }
+ }
+
+ return h_new_parser(mm__, &sequence_vt, rewrite);
+}
+
+HParser* h_drop_from___a(void *args[]) {
+ return h_drop_from___ma(&system_allocator, args);
+}
+
+HParser* h_drop_from___ma(HAllocator* mm__, void *args[]) {
+ HParser *p = (HParser*)(args[0]);
+ assert_message(p->vtable == &sequence_vt, "drop_from requires a sequence parser");
+ HSequence *s = (HSequence*)(p->env);
+ HSequence *rewrite = h_new(HSequence, 1);
+ rewrite->p_array = h_new(HParser *, s->len);
+ rewrite->len = s->len;
+
+ int i=0, *argp = (int*)(args[1]);
+ while (*argp >= 0) {
+ if (i == *argp) {
+ rewrite->p_array[i] = h_ignore(s->p_array[i]);
+ ++argp;
+ } else {
+ rewrite->p_array[i] = s->p_array[i];
+ }
+ ++i;
+ }
+
+ return h_new_parser(mm__, &sequence_vt, rewrite);
+}
diff --git a/src/t_parser.c b/src/t_parser.c
index 7e4ff8293990adbb642a5b7b54f8b743466a2654..cb67901ed9227787d5580079112c410df000dd94 100644
--- a/src/t_parser.c
+++ b/src/t_parser.c
@@ -845,6 +845,19 @@ static void test_seek(gconstpointer backend) {
g_check_parse_failed(p, be, "abc", 3);
}
+static void test_drop_from(gconstpointer backend) {
+ HParserBackend be = (HParserBackend)GPOINTER_TO_INT(backend);
+ HParser *p, *q, *r, *seq;
+
+ seq = h_sequence(h_ch('a'), h_ch('b'), h_ch('c'), h_ch('d'), h_ch('e'), NULL);
+ p = h_drop_from(seq, 0, 4);
+ g_check_parse_match(p, be, "abcde", 5, "(u0x62 u0x63 u0x64)");
+ //q = h_drop_from(seq, 1, 2, -1);
+ //g_check_parse_match(q, be, "abcde", 5, "(u0x61 u0x64 u0x65)");
+ //r = h_drop_from(seq, 0, 1, 3, 4, -1);
+ //g_check_parse_match(r, be, "abcde", 5, "(u0x63)");
+}
+
void register_parser_tests(void) {
g_test_add_data_func("/core/parser/packrat/token", GINT_TO_POINTER(PB_PACKRAT), test_token);
g_test_add_data_func("/core/parser/packrat/ch", GINT_TO_POINTER(PB_PACKRAT), test_ch);
@@ -902,6 +915,7 @@ void register_parser_tests(void) {
g_test_add_data_func("/core/parser/packrat/skip", GINT_TO_POINTER(PB_PACKRAT), test_skip);
g_test_add_data_func("/core/parser/packrat/seek", GINT_TO_POINTER(PB_PACKRAT), test_seek);
g_test_add_data_func("/core/parser/packrat/tell", GINT_TO_POINTER(PB_PACKRAT), test_tell);
+ g_test_add_data_func("/core/parser/packrat/drop_from", GINT_TO_POINTER(PB_PACKRAT), test_drop_from);
g_test_add_data_func("/core/parser/llk/token", GINT_TO_POINTER(PB_LLk), test_token);
g_test_add_data_func("/core/parser/llk/ch", GINT_TO_POINTER(PB_LLk), test_ch);
@@ -948,6 +962,7 @@ void register_parser_tests(void) {
g_test_add_data_func("/core/parser/llk/iterative", GINT_TO_POINTER(PB_LLk), test_iterative);
g_test_add_data_func("/core/parser/llk/iterative/lookahead", GINT_TO_POINTER(PB_LLk), test_iterative_lookahead);
g_test_add_data_func("/core/parser/llk/iterative/result_length", GINT_TO_POINTER(PB_LLk), test_iterative_result_length);
+ g_test_add_data_func("/core/parser/llk/drop_from", GINT_TO_POINTER(PB_LLk), test_drop_from);
g_test_add_data_func("/core/parser/regex/token", GINT_TO_POINTER(PB_REGULAR), test_token);
g_test_add_data_func("/core/parser/regex/ch", GINT_TO_POINTER(PB_REGULAR), test_ch);
@@ -989,6 +1004,7 @@ void register_parser_tests(void) {
g_test_add_data_func("/core/parser/regex/ignore", GINT_TO_POINTER(PB_REGULAR), test_ignore);
g_test_add_data_func("/core/parser/regex/result_length", GINT_TO_POINTER(PB_REGULAR), test_result_length);
g_test_add_data_func("/core/parser/regex/token_position", GINT_TO_POINTER(PB_REGULAR), test_token_position);
+ g_test_add_data_func("/core/parser/regex/drop_from", GINT_TO_POINTER(PB_REGULAR), test_drop_from);
g_test_add_data_func("/core/parser/lalr/token", GINT_TO_POINTER(PB_LALR), test_token);
g_test_add_data_func("/core/parser/lalr/ch", GINT_TO_POINTER(PB_LALR), test_ch);
@@ -1036,6 +1052,7 @@ void register_parser_tests(void) {
g_test_add_data_func("/core/parser/lalr/iterative", GINT_TO_POINTER(PB_LALR), test_iterative);
g_test_add_data_func("/core/parser/lalr/iterative/lookahead", GINT_TO_POINTER(PB_LALR), test_iterative_lookahead);
g_test_add_data_func("/core/parser/lalr/iterative/result_length", GINT_TO_POINTER(PB_LALR), test_iterative_result_length);
+ g_test_add_data_func("/core/parser/lalr/drop_from", GINT_TO_POINTER(PB_LALR), test_drop_from);
g_test_add_data_func("/core/parser/glr/token", GINT_TO_POINTER(PB_GLR), test_token);
g_test_add_data_func("/core/parser/glr/ch", GINT_TO_POINTER(PB_GLR), test_ch);
@@ -1081,4 +1098,5 @@ void register_parser_tests(void) {
g_test_add_data_func("/core/parser/glr/ambiguous", GINT_TO_POINTER(PB_GLR), test_ambiguous);
g_test_add_data_func("/core/parser/glr/result_length", GINT_TO_POINTER(PB_GLR), test_result_length);
g_test_add_data_func("/core/parser/glr/token_position", GINT_TO_POINTER(PB_GLR), test_token_position);
+ g_test_add_data_func("/core/parser/glr/drop_from", GINT_TO_POINTER(PB_GLR), test_drop_from);
}