Support for direct tuple member access

As discussed in #549

NOTE: This implements only the direct tuple member access (syntax `foo.N` with N a
number)

- It seems more efficient to wait for the general pattern-matching rewrite to
  handle pattern-matching on tuples
- Until then we keep the (now obsolete) `let (x, y) = pair in x` syntax, to
  leave time for updates, but we won't be documenting it

compiler/desugared/from_surface.ml

@@ -715,6 +715,8 @@ let rec translate_expr
     Expr.ematch ~e:(rec_helper e1) ~name:enum_uid ~cases emark
   | ArrayLit es -> Expr.earray (List.map rec_helper es) emark
   | Tuple es -> Expr.etuple (List.map rec_helper es) emark
+  | TupleAccess (e, n) ->
+    Expr.etupleaccess ~e:(rec_helper e) ~index:(Mark.remove n - 1) ~size:0 emark
   | CollectionOp (((S.Filter { f } | S.Map { f }) as op), collection) ->
     let param_names, predicate = f in
     let collection =

compiler/shared_ast/expr.ml

@@ -117,7 +117,7 @@ let eexternal ~name mark = Mark.add mark (Bindlib.box (EExternal { name }))
 let etuple args = Box.appn args @@ fun args -> ETuple args
 
 let etupleaccess ~e ~index ~size =
-  assert (index < size);
+  assert (size = 0 || index < size);
   Box.app1 e @@ fun e -> ETupleAccess { e; index; size }
 
 let earray args = Box.appn args @@ fun args -> EArray args

compiler/shared_ast/typing.ml

@@ -799,16 +799,36 @@ and typecheck_expr_top_down :
     let es' = List.map2 (typecheck_expr_top_down ctx env) tys es in
     Expr.etuple es' mark
   | A.ETupleAccess { e = e1; index; size } ->
-    if index >= size then
-      Message.error ~pos:(Expr.pos e) "Tuple access out of bounds (%d/%d)" index
-        size;
+    let out_of_bounds size =
+      Message.error ~pos:pos_e "Tuple access out of bounds (%d/%d)" index size
+    in
     let tuple_ty =
+      if size = 0 then (* Unset yet, we resolve it now *)
+        TAny (Any.fresh ())
+      else if index >= size then out_of_bounds size
+      else
         TTuple
           (List.init size (fun n ->
-             if n = index then tau else unionfind ~pos:e1 (TAny (Any.fresh ()))))
+               if n = index then tau
+               else unionfind ~pos:e1 (TAny (Any.fresh ()))))
     in
-    let e1' = typecheck_expr_top_down ctx env (unionfind ~pos:e1 tuple_ty) e1 in
-    Expr.etupleaccess ~e:e1' ~index ~size context_mark
+    let tuple_ty = unionfind ~pos:e1 tuple_ty in
+    let e1' = typecheck_expr_top_down ctx env tuple_ty e1 in
+    let size, mark =
+      if size <> 0 then size, context_mark
+      else
+        match typ_to_ast ~flags tuple_ty with
+        | TTuple l, _ -> (
+          match List.nth_opt l index with
+          | None -> out_of_bounds (List.length l)
+          | Some ty -> List.length l, mark_with_tau_and_unify (ast_to_typ ty))
+        | TAny, _ -> failwith "Disambiguation failure"
+        | ty ->
+          Message.error ~pos:(Expr.pos e1)
+            "This expression has type@ %a,@ while a tuple was expected"
+            (Print.typ ctx) ty
+    in
+    Expr.etupleaccess ~e:e1' ~index ~size mark
   | A.EAbs { binder; tys = t_args } ->
     if Bindlib.mbinder_arity binder <> List.length t_args then
       Message.error ~pos:(Expr.pos e)

compiler/shared_ast/typing.mli

@@ -70,6 +70,9 @@ val expr :
     - disambiguation of constructors: [EDStructAccess] nodes are translated into
       [EStructAccess] with the suitable structure and field idents (this only
       concerns [desugared] expressions).
+    - disambiguation of structure names in [EDStructAmend] nodes ([desugared] as
+      well)
+    - resolution of tuple size (when equal to 0) on [ETupleAccess] nodes
     - resolution of operator types, which are stored (monomorphised) back in the
       [EAppOp] nodes
     - resolution of function application input types on the [EApp] nodes, when

compiler/surface/ast.ml

@@ -194,6 +194,7 @@ and naked_expression =
   (* path, ident, state *)
   | Dotted of expression * (path * lident Mark.pos) Mark.pos
       (** Dotted is for both struct field projection and sub-scope variables *)
+  | TupleAccess of expression * int Mark.pos
 
 and exception_to =
   | NotAnException

compiler/surface/parser.mly

@@ -191,6 +191,13 @@ let naked_expression ==
 }
 | e = expression ;
   DOT ; i = addpos(qlident) ; <Dotted>
+| e = expression ; DOT ; arg = addpos(INT_LITERAL) ; {
+  let n_str, pos_n = arg in
+  let n = int_of_string n_str in
+  if n <= 0 then
+    Message.error ~pos:pos_n "Tuple indices must be >= 1";
+  TupleAccess (e, (n, pos_n))
+}
 | CARDINAL ; {
   Builtin Cardinal
 }

compiler/surface/parser_driver.ml

@@ -133,6 +133,7 @@ module ParserAux (LocalisedLexer : Lexer_common.LocalisedLexer) = struct
       | msg ->
         Format.fprintf ppf "@{<yellow>@<1>»@} @[<hov>%a@]" Format.pp_print_text
           (String.trim (String.uncapitalize_ascii msg)));
+      if acceptable_tokens <> [] then
         Format.fprintf ppf "@,@[<hov>Those are valid at this point:@ %a@]"
           (Format.pp_print_list
              ~pp_sep:(fun ppf () -> Format.fprintf ppf ",@ ")

tests/tuples/good/tuples.catala_en

@@ -16,7 +16,7 @@ declaration f2 content decimal
   equals
   match en with pattern
   -- One of str1:
-     let (a, w) equals str.x1 in
+     let a equals str.x1.1 in
      let (b, w) equals str1.x1 in
      a / b
   -- Two of z:

tests/tuples/good/tuplists.catala_en

@@ -37,8 +37,7 @@ scope S:
   definition r5 equals (x * y, y / z) for (x, y, z) among (lis1, lis2, lis3)
   definition r6 equals
     let lis12 equals (x, y) for (x, y) among (lis1, lis2) in
-    (let (x, y) equals xy in (x * y, y / z))
-      for (xy, z) among (lis12, lis3)
+    (xy.1 * xy.2, xy.2 / z) for (xy, z) among (lis12, lis3)
 
 ```