Add return types to procedures (#1197)

This PR introduces the `Return`  statement in the AST present with the `_return :=` construction in concrete syntax:

```
procedure foo() -> String
  a = my_string;
  _return := a;
end
```

Design decisions in the current implementation:
* `_return` takes as an argument a single variable with the return value. It seems natural to use this approach in the ANF-based language.
* Don't support empty return statements (`_return`) for early exit when the procedure has no a return type. e.g. the following code is forbidden:

```
procedure no_return()
  match something with
  | 0 => (* do something *)
  | _ => _return; (* typechecking error: cannot use an empty return for early exit *)
  end
end
```

Notes:
*  Gas cost of the return call is 1. 
*  We use cram tests to check the typechecker because of #1196

Closes #578

Makefile

@@ -66,7 +66,7 @@ parser-messages:
 	mv src/base/NewParserFaults.messages src/base/ParserFaults.messages
 	rm src/base/NewParserFaultsStubs.messages
 
-# Launch utop such that it finds the libraroes.
+# Launch utop such that it finds the libraries.
 utop: release
 	OCAMLPATH=_build/install/default/lib:$(OCAMLPATH) utop
 

src/base/Accept.ml

@@ -111,8 +111,9 @@ struct
                 @ List.fold_left stmts ~init:[] ~f:(fun acc s ->
                       acc @ walk_stmt s))
         | Load _ | RemoteLoad _ | Store _ | MapUpdate _ | MapGet _
-        | RemoteMapGet _ | ReadFromBC _ | TypeCast _ | AcceptPayment | Iterate _
-        | SendMsgs _ | CreateEvnt _ | CallProc _ | Throw _ | GasStmt _ ->
+        | RemoteMapGet _ | ReadFromBC _ | TypeCast _ | AcceptPayment | Return _
+        | Iterate _ | SendMsgs _ | CreateEvnt _ | CallProc _ | Throw _
+        | GasStmt _ ->
             []
       in
       List.fold_left comp.comp_body ~init:[] ~f:(fun acc s -> acc @ walk_stmt s)

src/base/Callgraph.ml

@@ -189,8 +189,8 @@ module ScillaCallgraph (SR : Rep) (ER : Rep) = struct
     let rec visit_stmt (s, _annot) =
       match s with
       | Bind (_id, ea) -> collect_funcalls ea collected_nodes
-      | CallProc (id, _) | Iterate (_, id) -> (
-          find_node collected_nodes id |> function
+      | CallProc (_, proc, _) | Iterate (_, proc) -> (
+          find_node collected_nodes proc |> function
           | Some n -> NodeSet.singleton n
           | None -> emp_nodes_set)
       | MatchStmt (_id, arms) ->
@@ -200,8 +200,8 @@ module ScillaCallgraph (SR : Rep) (ER : Rep) = struct
                   NodeSet.union acc @@ visit_stmt sa)
               |> NodeSet.union acc)
       | Load _ | RemoteLoad _ | Store _ | MapUpdate _ | MapGet _
-      | RemoteMapGet _ | ReadFromBC _ | TypeCast _ | AcceptPayment | SendMsgs _
-      | CreateEvnt _ | Throw _ | GasStmt _ ->
+      | RemoteMapGet _ | ReadFromBC _ | TypeCast _ | AcceptPayment | Return _
+      | SendMsgs _ | CreateEvnt _ | Throw _ | GasStmt _ ->
           emp_nodes_set
     in
     List.fold_left comp.comp_body ~init:emp_nodes_set ~f:(fun acc s ->

src/base/Cashflow.ml

@@ -227,10 +227,12 @@ struct
       | TypeCast (x, r, t) ->
           CFSyntax.TypeCast (add_noinfo_to_ident x, add_noinfo_to_ident r, t)
       | AcceptPayment -> CFSyntax.AcceptPayment
+      | Return i -> CFSyntax.Return (add_noinfo_to_ident i)
       | SendMsgs x -> CFSyntax.SendMsgs (add_noinfo_to_ident x)
       | CreateEvnt x -> CFSyntax.CreateEvnt (add_noinfo_to_ident x)
-      | CallProc (p, args) ->
-          CFSyntax.CallProc (p, List.map args ~f:add_noinfo_to_ident)
+      | CallProc (id_opt, p, args) ->
+          let id = Option.map id_opt ~f:(fun id -> add_noinfo_to_ident id) in
+          CFSyntax.CallProc (id, p, List.map args ~f:add_noinfo_to_ident)
       | Iterate (l, p) -> CFSyntax.Iterate (add_noinfo_to_ident l, p)
       | Throw xopt -> (
           match xopt with
@@ -241,13 +243,16 @@ struct
     (res_s, rep)
 
   let cf_init_tag_component component =
-    let { comp_type; comp_name; comp_params; comp_body } = component in
+    let { comp_type; comp_name; comp_params; comp_body; comp_return } =
+      component
+    in
     {
       CFSyntax.comp_type;
       CFSyntax.comp_name;
       CFSyntax.comp_params =
         List.map ~f:(fun (x, t) -> (add_noinfo_to_ident x, t)) comp_params;
       CFSyntax.comp_body = List.map ~f:cf_init_tag_stmt comp_body;
+      CFSyntax.comp_return;
     }
 
   let cf_init_tag_contract contract token_fields =
@@ -1870,6 +1875,18 @@ struct
               || [%equal: ECFR.money_tag] (get_id_tag r) r_tag) )
       | AcceptPayment ->
           (AcceptPayment, param_env, field_env, local_env, ctr_tag_map, false)
+      | Return i ->
+          let i_tag = lub_tags NoInfo (lookup_var_tag2 i local_env param_env) in
+          let new_i = update_id_tag i i_tag in
+          let new_local_env, new_param_env =
+            update_var_tag2 i i_tag local_env param_env
+          in
+          ( Return new_i,
+            new_param_env,
+            field_env,
+            new_local_env,
+            ctr_tag_map,
+            not @@ [%equal: ECFR.money_tag] (get_id_tag i) i_tag )
       | GasStmt g ->
           (GasStmt g, param_env, field_env, local_env, ctr_tag_map, false)
       | SendMsgs m ->
@@ -1896,7 +1913,9 @@ struct
             new_local_env,
             ctr_tag_map,
             not @@ [%equal: ECFR.money_tag] (get_id_tag e) e_tag )
-      | CallProc (p, args) ->
+      | CallProc (id_opt, p, args) ->
+          (* TODO: Bindings from procedure calls are not taken into account in
+                   the cash flow analysis. *)
           let new_args =
             List.map args ~f:(fun arg ->
                 update_id_tag arg (lookup_var_tag2 arg local_env param_env))
@@ -1913,7 +1932,7 @@ struct
             | Ok res -> res
             | Unequal_lengths -> false
           in
-          ( CallProc (p, new_args),
+          ( CallProc (id_opt, p, new_args),
             param_env,
             field_env,
             local_env,
@@ -1991,7 +2010,7 @@ struct
           new_changes || acc_changes ))
 
   let cf_tag_component t param_env field_env ctr_tag_map =
-    let { comp_type; comp_name; comp_params; comp_body } = t in
+    let { comp_type; comp_name; comp_params; comp_body; comp_return } = t in
     let empty_local_env = AssocDictionary.make_dict () in
     let implicit_local_env =
       AssocDictionary.insert MessagePayload.amount_label Money
@@ -2026,6 +2045,7 @@ struct
         comp_name;
         comp_params = new_params;
         comp_body = new_comp_body;
+        comp_return;
       },
       new_param_env,
       new_field_env,

src/base/Checker.ml

@@ -63,10 +63,10 @@ module CG = ScillaCallgraph (TCSRep) (TCERep)
 
 (* Check that the module parses *)
 let check_parsing ctr syn =
-  let cmod = FEParser.parse_file syn ctr in
-  if Result.is_ok cmod then
+  let ast = FEParser.parse_file syn ctr in
+  if Result.is_ok ast then
     plog @@ sprintf "\n[Parsing]:\n module [%s] is successfully parsed.\n" ctr;
-  cmod
+  ast
 
 (* Change local names to global names *)
 let disambiguate_lmod lmod elibs names_and_addresses this_address =
@@ -309,6 +309,7 @@ let check_cmodule cli =
       wrap_error_with_gas initial_gas
       @@ check_parsing cli.input_file Parser.Incremental.cmodule
     in
+    let cmod = FEParser.disambiguate_calls cmod in
     (* Import whatever libs we want. *)
     let this_address_opt, init_address_map =
       Option.value_map cli.init_file ~f:get_init_this_address_and_extlibs

src/base/DeadCodeDetector.ml

@@ -255,8 +255,9 @@ module DeadCodeDetector (SR : Rep) (ER : Rep) = struct
       | Bind (_, e) -> report_expr e
       | MatchStmt (_, pslist) -> List.iter pslist ~f:report_unreachable_adapter
       | Load _ | RemoteLoad _ | Store _ | MapUpdate _ | MapGet _
-      | RemoteMapGet _ | ReadFromBC _ | TypeCast _ | AcceptPayment | GasStmt _
-      | Throw _ | Iterate _ | CallProc _ | CreateEvnt _ | SendMsgs _ ->
+      | RemoteMapGet _ | ReadFromBC _ | TypeCast _ | AcceptPayment | Return _
+      | GasStmt _ | Throw _ | Iterate _ | CallProc _ | CreateEvnt _ | SendMsgs _
+        ->
           ()
     in
     Option.iter cmod.libs ~f:(fun l ->
@@ -424,17 +425,25 @@ module DeadCodeDetector (SR : Rep) (ER : Rep) = struct
             match topt with
             | Some x -> (ERSet.add lv x, adts, ctrs)
             | None -> (lv, adts, ctrs))
-        | CallProc (p, al) ->
+        | CallProc (id_opt, p, al) ->
             proc_dict := p :: !proc_dict;
-            (ERSet.of_list al |> ERSet.union lv, adts, ctrs)
+            let lv' =
+              match id_opt with
+              | Some id when ERSet.mem lv id -> ERSet.add lv id
+              | Some id ->
+                  warn "Unused local binding: " id ER.get_loc;
+                  ERSet.add lv id
+              | None -> lv
+            in
+            (ERSet.of_list al |> ERSet.union lv', adts, ctrs)
         | Iterate (l, p) ->
             proc_dict := p :: !proc_dict;
             (ERSet.add lv l, adts, ctrs)
         | Bind (i, e) ->
-            let live_vars_no_i =
-              ERSet.filter ~f:(fun x -> not @@ SCIdentifier.equal i x) lv
-            in
             if ERSet.mem lv i then
+              let live_vars_no_i =
+                ERSet.filter ~f:(fun x -> not @@ SCIdentifier.equal i x) lv
+              in
               let e_live_vars, adts', ctrs' = expr_iter e in
               ( ERSet.union e_live_vars live_vars_no_i,
                 SCIdentifierSet.union adts' adts,
@@ -485,7 +494,7 @@ module DeadCodeDetector (SR : Rep) (ER : Rep) = struct
             else (
               warn "Unused type cast statement to: " x ER.get_loc;
               (ERSet.add lv r, adts, ctrs))
-        | SendMsgs v | CreateEvnt v -> (ERSet.add lv v, adts, ctrs)
+        | SendMsgs v | CreateEvnt v | Return v -> (ERSet.add lv v, adts, ctrs)
         | AcceptPayment | GasStmt _ -> (lv, adts, ctrs))
     | _ -> (emp_erset, emp_idset, emp_idset)
 
@@ -543,8 +552,8 @@ module DeadCodeDetector (SR : Rep) (ER : Rep) = struct
                 get_used_address_fields address_params sa |> merge_id_maps m)
             |> merge_id_maps m)
     | Bind _ | Load _ | Store _ | MapUpdate _ | MapGet _ | ReadFromBC _
-    | TypeCast _ | AcceptPayment | Iterate _ | SendMsgs _ | CreateEvnt _
-    | CallProc _ | Throw _ | GasStmt _ ->
+    | TypeCast _ | AcceptPayment | Return _ | Iterate _ | SendMsgs _
+    | CreateEvnt _ | CallProc _ | Throw _ | GasStmt _ ->
         emp_idsmap
 
   (** Returns a set of field names of the contract address type. *)
@@ -789,8 +798,8 @@ module DeadCodeDetector (SR : Rep) (ER : Rep) = struct
               Map.set used ~key:ctr_name ~data:ctr_arg_pos_to_fields'
           | _ -> used)
       | Bind _ | Load _ | Store _ | MapUpdate _ | MapGet _ | ReadFromBC _
-      | TypeCast _ | AcceptPayment | Iterate _ | SendMsgs _ | CreateEvnt _
-      | CallProc _ | Throw _ | GasStmt _ ->
+      | TypeCast _ | AcceptPayment | Return _ | Iterate _ | SendMsgs _
+      | CreateEvnt _ | CallProc _ | Throw _ | GasStmt _ ->
           used
     in
     List.fold_left comp.comp_body ~init:used ~f:(fun used s -> aux used s)

src/base/Disambiguate.ml

@@ -678,6 +678,11 @@ module ScillaDisambiguation (SR : Rep) (ER : Rep) = struct
             in
             pure @@ (PostDisSyntax.TypeCast (dis_x, dis_r, dis_t), new_var_dict)
         | AcceptPayment -> pure @@ (PostDisSyntax.AcceptPayment, var_dict_acc)
+        | Return i ->
+            let%bind dis_i =
+              disambiguate_identifier_helper var_dict_acc (SR.get_loc rep) i
+            in
+            pure @@ (PostDisSyntax.Return dis_i, var_dict_acc)
         | Iterate (l, proc) ->
             let%bind dis_l =
               disambiguate_identifier_helper var_dict_acc (SR.get_loc rep) l
@@ -695,15 +700,24 @@ module ScillaDisambiguation (SR : Rep) (ER : Rep) = struct
               disambiguate_identifier_helper var_dict_acc (SR.get_loc rep) e
             in
             pure @@ (PostDisSyntax.CreateEvnt dis_e, var_dict_acc)
-        | CallProc (proc, args) ->
+        | CallProc (id_opt, proc, args) ->
+            let%bind dis_id_opt =
+              match id_opt with
+              | Some id ->
+                  let%bind dis_id = name_def_as_simple_global id in
+                  pure @@ Some dis_id
+              | None -> pure @@ None
+            in
             (* Only locally defined procedures are allowed *)
             let%bind dis_proc = name_def_as_simple_global proc in
             let%bind dis_args =
               mapM args
                 ~f:
                   (disambiguate_identifier_helper var_dict_acc (SR.get_loc rep))
             in
-            pure @@ (PostDisSyntax.CallProc (dis_proc, dis_args), var_dict_acc)
+            pure
+            @@ ( PostDisSyntax.CallProc (dis_id_opt, dis_proc, dis_args),
+                 var_dict_acc )
         | Throw xopt ->
             let%bind dis_xopt =
               option_mapM xopt
@@ -731,7 +745,7 @@ module ScillaDisambiguation (SR : Rep) (ER : Rep) = struct
   (**************************************************************)
 
   let disambiguate_component (dicts : name_dicts) comp =
-    let { comp_type; comp_name; comp_params; comp_body } = comp in
+    let { comp_type; comp_name; comp_params; comp_body; comp_return } = comp in
     let%bind dis_comp_name = name_def_as_simple_global comp_name in
     let%bind dis_comp_params =
       mapM comp_params ~f:(fun (x, t) ->
@@ -746,13 +760,21 @@ module ScillaDisambiguation (SR : Rep) (ER : Rep) = struct
           remove_local_id_from_dict dict (as_string x))
     in
     let body_dicts = { dicts with var_dict = body_var_dict } in
+    let%bind dis_return =
+      match comp_return with
+      | None -> pure None
+      | Some ret ->
+          let%bind dis_t = disambiguate_type dicts.typ_dict ret in
+          pure (Some dis_t)
+    in
     let%bind dis_comp_body = disambiguate_stmts body_dicts comp_body in
     pure
     @@ {
          PostDisSyntax.comp_type;
          PostDisSyntax.comp_name = dis_comp_name;
          PostDisSyntax.comp_params = dis_comp_params;
          PostDisSyntax.comp_body = dis_comp_body;
+         PostDisSyntax.comp_return = dis_return;
        }
 
   (**************************************************************)

src/base/FrontEndParser.ml

@@ -32,10 +32,61 @@ module ScillaFrontEndParser (Literal : ScillaLiteral) = struct
   module Lexer = ScillaLexer.MkLexer (FESyntax)
   module MInter = Parser.MenhirInterpreter
   module FEPType = FESyntax.SType
+  module FEPIdentifier = FEPType.TIdentifier
+
+  module FEPIdentifierComp = struct
+    include FEPIdentifier.Name
+    include Comparable.Make (FEPIdentifier.Name)
+  end
+
+  module FEPIdentifierSet = Set.Make (FEPIdentifierComp)
+
+  let emp_idset = FEPIdentifierSet.empty
 
   (* TODO: Use DebugMessage perr/pout instead of fprintf. *)
   let fail_err msg lexbuf = fail1 ~kind:msg ?inst:None (toLoc lexbuf.lex_curr_p)
 
+  (** Disambiguates calls of procedures without values and pure function calls
+      and variables.
+      They have the same syntax: [id = proc param1 param2] or [id = proc].
+      Therefore, the parser doesn't know what is actually is called and saves
+      such cases as [Bind(id, App(proc, [param1, param2]))] or
+      [Bind(id, Var(proc)].
+      This function finishes parsing and places [CallProc(Some(id), ...)]
+      statements when the contract contains a procedure [id]. *)
+  let disambiguate_calls cmod =
+    let open FESyntax in
+    let procedures_with_return =
+      List.fold_left cmod.contr.ccomps ~init:emp_idset ~f:(fun s comp ->
+          if Option.is_some comp.comp_return then
+            FEPIdentifierSet.add s (FEPIdentifier.get_id comp.comp_name)
+          else s)
+    in
+    let disambiguate_stmt (stmt, annot) =
+      match stmt with
+      | Bind (id, (App (f, args), _))
+        when Set.mem procedures_with_return (FEPIdentifier.get_id f) ->
+          (CallProc (Some id, f, args), annot)
+      | Bind (id, (Var f, _))
+        when Set.mem procedures_with_return (FEPIdentifier.get_id f) ->
+          (CallProc (Some id, f, []), annot)
+      | _ -> (stmt, annot)
+    in
+    let contr' =
+      {
+        cmod.contr with
+        ccomps =
+          List.map cmod.contr.ccomps ~f:(fun comp ->
+              {
+                comp with
+                comp_body =
+                  List.map comp.comp_body ~f:(fun stmt ->
+                      disambiguate_stmt stmt);
+              });
+      }
+    in
+    { cmod with contr = contr' }
+
   let parse_lexbuf checkpoint_starter lexbuf filename =
     lexbuf.lex_curr_p <- { lexbuf.lex_curr_p with pos_fname = filename };
     (* Supply of tokens *)
@@ -86,6 +137,11 @@ module ScillaFrontEndParser (Literal : ScillaLiteral) = struct
 
   let parse_expr_from_stdin () = parse_stdin Parser.Incremental.exp_term
   let parse_lmodule filename = parse_file Parser.Incremental.lmodule filename
-  let parse_cmodule filename = parse_file Parser.Incremental.cmodule filename
+
+  let parse_cmodule filename =
+    let open Result.Let_syntax in
+    let%bind cmod = parse_file Parser.Incremental.cmodule filename in
+    pure @@ disambiguate_calls cmod
+
   let get_comments () = Lexer.get_comments ()
 end

src/base/Gas.ml

@@ -238,7 +238,7 @@ module ScillaGas (SR : Rep) (ER : Rep) = struct
               in
               let s' = MatchStmt (x, clauses') in
               pure @@ [ (GasStmt g, srep); (s', srep) ]
-          | AcceptPayment ->
+          | AcceptPayment | Return _ ->
               let g = GasStmt (GasGasCharge.StaticCost 1) in
               pure @@ [ (g, srep); (s, srep) ]
           | Iterate (l, _) ->