Merge pull request #913 from ishehadeh/feature/destructuring-rest

Support for `& rest` pattern in destructure and match

src/boot/boot.janet

@@ -1678,6 +1678,7 @@
 
   * array or bracket tuple -- an array or bracket tuple will match only if
     all of its elements match the corresponding elements in `x`.
+    Use `& rest` at the end of an array or bracketed tuple to bind all remaining values to `rest`.
 
   * table or struct -- a table or struct will match if all values match with
     the corresponding values in `x`.
@@ -1751,10 +1752,26 @@
       (break)
 
       # match data structure template
-      (or isarr (= t :struct) (= t :table))
+      (or (= t :struct) (= t :table))
-      (do
-        (when isarr (get-length-sym s))
       (eachp [i sub-pattern] pattern
+        (visit-pattern-1 b2g s i sub-pattern))
+
+      isarr
+      (do
+        (get-length-sym s)
+        (eachp [i sub-pattern] pattern
+          (when (= sub-pattern '&)
+            (when (<= (length pattern) (inc i))
+              (errorf "expected symbol following & in pattern"))
+
+            (when (< (+ i 2) (length pattern))
+              (errorf "expected a single symbol follow '& in pattern, found %q" (slice pattern (inc i))))
+
+            (when (not= (type (pattern (inc i))) :symbol)
+              (errorf "expected symbol following & in pattern, found %q" (pattern (inc i))))
+
+            (put b2g (pattern (inc i)) @[[slice s i]])
+            (break))
           (visit-pattern-1 b2g s i sub-pattern)))
 
       # match global unification
@@ -1774,14 +1791,24 @@
     (def isarr (or (= t :array) (and (= t :tuple) (= (tuple/type pattern) :brackets))))
     (when isarr
       (array/push anda (get-length-sym s))
-      (array/push anda [<= (length pattern) (get-length-sym s)]))
+      (def pattern-len
+        (if-let [ rest-idx (find-index (fn [x] (= x '&)) pattern) ]
+          rest-idx
+          (length pattern)))
+      (array/push anda [<= pattern-len (get-length-sym s)]))
     (cond
 
       # match data structure template
-      (or isarr (= t :struct) (= t :table))
+      (or (= t :struct) (= t :table))
       (eachp [i sub-pattern] pattern
-        (when (not isarr)
+        (array/push anda [not= nil (get-sym s i)])
-          (array/push anda [not= nil (get-sym s i)]))
+        (visit-pattern-2 anda gun preds s i sub-pattern))
+      
+      isarr
+      (eachp [i sub-pattern] pattern
+        # stop recursing to sub-patterns if the rest sigil is found
+        (when (= sub-pattern '&)
+          (break))
         (visit-pattern-2 anda gun preds s i sub-pattern))
 
       # match local binding

src/core/specials.c

@@ -154,6 +154,69 @@ static int destructure(JanetCompiler *c,
             for (int32_t i = 0; i < len; i++) {
                 JanetSlot nextright = janetc_farslot(c);
                 Janet subval = values[i];
+
+                if (janet_checktype(subval, JANET_SYMBOL) && !janet_cstrcmp(janet_unwrap_symbol(subval), "&")) {
+                    if (i + 1 >= len) {
+                        janetc_cerror(c, "expected symbol following '& in destructuring pattern");
+                        return 1;
+                    }
+
+                    if (i + 2 < len) {
+                        int32_t num_extra = len - i - 1;
+                        Janet* extra = janet_tuple_begin(num_extra);
+                        janet_tuple_flag(extra) |= JANET_TUPLE_FLAG_BRACKETCTOR;
+
+                        for (int32_t j = 0; j < num_extra; ++j) {
+                            extra[j] = values[j + i + 1];
+                        }
+
+                        janetc_error(c, janet_formatc("expected a single symbol follow '& in destructuring pattern, found %q", janet_wrap_tuple(janet_tuple_end(extra))));
+                        return 1;
+                    }
+
+
+                    if (!janet_checktype(values[i + 1], JANET_SYMBOL)) {
+                        janetc_error(c, janet_formatc("expected symbol following '& in destructuring pattern, found %q", values[i + 1]));
+                        return 1;
+                    }
+
+                    JanetSlot argi = janetc_farslot(c);
+                    JanetSlot arg  = janetc_farslot(c);
+                    JanetSlot len  = janetc_farslot(c);
+
+                    janetc_emit_si(c, JOP_LOAD_INTEGER, argi, i, 0);
+                    janetc_emit_ss(c, JOP_LENGTH, len, right, 0);
+
+                    // loop condition
+                    // reuse arg slot for the condition result
+                    int32_t label_loop_start = janetc_emit_sss(c, JOP_EQUALS, arg, argi, len, 0);
+                    int32_t label_loop_cond_jump = janetc_emit_si(c, JOP_JUMP_IF, arg, 0, 0);
+
+                    // loop body
+                    janetc_emit_sss(c, JOP_GET, arg, right, argi, 0);
+                    janetc_emit_s(c, JOP_PUSH, arg, 0);
+                    janetc_emit_ssi(c, JOP_ADD_IMMEDIATE, argi, argi, 1, 0);
+
+                    // loop
+                    // jump back to the start of the loop
+                    int32_t label_loop_loop = janet_v_count(c->buffer);
+                    janetc_emit(c, JOP_JUMP);
+                    int32_t label_loop_exit = janet_v_count(c->buffer);
+
+                    c->buffer[label_loop_cond_jump] |= (label_loop_exit - label_loop_cond_jump) << 16;
+                    c->buffer[label_loop_loop] |= (label_loop_start - label_loop_loop) << 8;
+
+                    janetc_freeslot(c, argi);
+                    janetc_freeslot(c, arg);
+                    janetc_freeslot(c, len);
+
+                    janetc_emit_s(c, JOP_MAKE_TUPLE, nextright, 1);
+
+                    leaf(c, janet_unwrap_symbol(values[i + 1]), nextright, attr);
+                    janetc_freeslot(c, nextright);
+                    break;
+                }
+
                 if (i < 0x100) {
                     janetc_emit_ssu(c, JOP_GET_INDEX, nextright, right, (uint8_t) i, 1);
                 } else {

test/suite0001.janet

@@ -137,6 +137,39 @@
   (assert (= a 1) "dictionary destructuring 3")
   (assert (= b 2) "dictionary destructuring 4")
   (assert (= c 4) "dictionary destructuring 5 - expression as key"))
+(let [test-tuple [:a :b 1 2]]
+  (def [a b one two] test-tuple)
+  (assert (= a :a) "tuple destructuring 1")
+  (assert (= b :b) "tuple destructuring 2")
+  (assert (= two 2) "tuple destructuring 3"))
+(let [test-tuple [:a :b 1 2]]
+  (def [a & rest] test-tuple)
+  (assert (= a :a) "tuple destructuring 4 - rest")
+  (assert (= rest [:b 1 2]) "tuple destructuring 5 - rest"))
+(do
+  (def [a b & rest] [:a :b nil :d])
+  (assert (= a :a) "tuple destructuring 6 - rest")
+  (assert (= b :b) "tuple destructuring 7 - rest")
+  (assert (= rest [nil :d]) "tuple destructuring 8 - rest"))
+(do
+  (def [[a b] x & rest] [[1 2] :a :c :b :a])
+  (assert (= a 1) "tuple destructuring 9 - rest")
+  (assert (= b 2) "tuple destructuring 10 - rest")
+  (assert (= x :a) "tuple destructuring 11 - rest")
+  (assert (= rest [:c :b :a]) "tuple destructuring 12 - rest"))
+(do
+  (def [a b & rest] [:a :b])
+  (assert (= a :a) "tuple destructuring 13 - rest")
+  (assert (= b :b) "tuple destructuring 14 - rest")
+  (assert (= rest []) "tuple destructuring 15 - rest"))
+
+(do
+  (def [[a b & r1] c & r2] [[:a :b 1 2] :c 3 4])
+  (assert (= a :a) "tuple destructuring 16 - rest")
+  (assert (= b :b) "tuple destructuring 17 - rest")
+  (assert (= c :c) "tuple destructuring 18 - rest")
+  (assert (= r1 [1 2]) "tuple destructuring 19 - rest")
+  (assert (= r2 [3 4]) "tuple destructuring 20 - rest"))
 
 # Marshal
 

test/suite0008.janet

@@ -106,6 +106,10 @@
 (assert (= nil (match {:a :hi} {:a a :b b} a)) "match 3")
 (assert (= nil (match [1 2] [a b c] a)) "match 4")
 (assert (= 2 (match [1 2] [a b] b)) "match 5")
+(assert (= [2 :a :b] (match [1 2 :a :b] [o & rest] rest)) "match 6")
+(assert (= [] (match @[:a] @[x & r] r :fallback)) "match 7")
+(assert (= :fallback (match @[1] @[x y & r] r :fallback)) "match 8")
+(assert (= [1 2 3 4] (match @[1 2 3 4] @[x y z & r] [x y z ;r] :fallback)) "match 9")
 
 # And/or checks