Parser: add syntax "(e1.f1, e2.f2) <- (v1, v2)" for parallel assignments

src/parser/parser.mly

@@ -660,11 +660,22 @@ expr_:
 | IF seq_expr THEN expr %prec prec_no_else
     { Eif ($2, $4, mk_expr (Etuple []) $startpos $endpos) }
 | expr LARROW expr
-    { match $1.expr_desc with
-      | Eidapp (q, [e1]) -> Eassign (e1, q, $3)
-      | Eidapp (Qident id, [e1;e2]) when id.id_str = mixfix "[]" ->
-          Eidapp (Qident {id with id_str = mixfix "[]<-"}, [e1;e2;$3])
-      | _ -> raise Error }
+    { let loc = floc $startpos $endpos in
+      let rec down ll rl = match ll, rl with
+        | {expr_desc = Eidapp (q, [e1])}::ll, e2::rl -> (e1,q,e2) :: down ll rl
+        | {expr_desc = Eidapp (Qident id, [_;_]); expr_loc = loc}::_, _::_
+          when id.id_str = mixfix "[]" -> Loc.errorm ~loc
+            "Parallel array assignments are not allowed"
+        | {expr_loc = loc}::_, _::_ -> Loc.errorm ~loc
+            "Invalid left expression in an assignment"
+        | [], [] -> []
+        | _ -> Loc.errorm ~loc "Invalid parallel assignment" in
+      match $1.expr_desc, $3.expr_desc with
+        | Eidapp (Qident id, [e1;e2]), _ when id.id_str = mixfix "[]" ->
+            Eidapp (Qident {id with id_str = mixfix "[]<-"}, [e1;e2;$3])
+        | Etuple ll, Etuple rl -> Eassign (down ll rl)
+        | Etuple _, _ -> Loc.errorm ~loc "Invalid parallel assignment"
+        | _, _ -> Eassign (down [$1] [$3]) }
 | LET ghost kind pattern EQUAL seq_expr IN seq_expr
     { match $4.pat_desc with
       | Pvar id -> Elet (id, $2, $3, $6, $8)

src/parser/ptree.ml

@@ -128,7 +128,7 @@ and expr_desc =
   | Etuple of expr list
   | Erecord of (qualid * expr) list
   | Eupdate of expr * (qualid * expr) list
-  | Eassign of expr * qualid * expr
+  | Eassign of (expr * qualid * expr) list
   (* control *)
   | Esequence of expr * expr
   | Eif of expr * expr * expr

src/parser/typing.ml

@@ -676,8 +676,10 @@ let rec dexpr muc denv {expr_desc = desc; expr_loc = loc} =
       let id = create_user_id id in
       let denv = denv_add_var denv id (dity_of_ity ity_int) in
       DEfor (id, efrom, dir, eto, inv, dexpr muc denv e1)
-  | Ptree.Eassign (e1, q, e2) ->
-      DEassign [dexpr muc denv e1, find_record_field muc q, dexpr muc denv e2]
+  | Ptree.Eassign asl ->
+      let mk_assign (e1,q,e2) =
+        dexpr muc denv e1, find_record_field muc q, dexpr muc denv e2 in
+      DEassign (List.map mk_assign asl)
   | Ptree.Eraise (q, e1) ->
       let xs = find_xsymbol muc q in
       let e1 = match e1 with