Add hole expression (#433)

* Add hole expressions

* Fix the issue of unreported holes

* Add tests

* New line in the end of the test file

* Update CHANGELOG

* Add hole expression to the docs

* Do not treat hole as a special type

* Update docs

* Update docs/sophia_features.md

Co-authored-by: Radosław Rowicki <35342116+radrow@users.noreply.github.com>

Co-authored-by: Radosław Rowicki <35342116+radrow@users.noreply.github.com>

src/aeso_ast_infer_types.erl

@@ -1843,6 +1843,10 @@ infer_expr(_Env, Body={contract_pubkey, As, _}) ->
     {typed, As, Body, Con};
 infer_expr(_Env, Body={id, As, "_"}) ->
     {typed, As, Body, fresh_uvar(As)};
+infer_expr(_Env, Body={id, As, "???"}) ->
+    T = fresh_uvar(As),
+    type_error({hole_found, As, T}),
+    {typed, As, Body, T};
 infer_expr(Env, Id = {Tag, As, _}) when Tag == id; Tag == qid ->
     {QName, Type} = lookup_name(Env, As, Id),
     {typed, As, QName, Type};
@@ -3396,6 +3400,9 @@ mk_error({cannot_unify, A, B, Cxt, When}) ->
                         [pp(instantiate(A)), pp(instantiate(B))]),
     {Pos, Ctxt} = pp_when(When),
     mk_t_err(Pos, Msg, Ctxt);
+mk_error({hole_found, Ann, Type}) ->
+    Msg = io_lib:format("Found a hole of type `~s`", [pp(instantiate(Type))]),
+    mk_t_err(pos(Ann), Msg);
 mk_error({unbound_variable, Id}) ->
     Msg = io_lib:format("Unbound variable `~s`", [pp(Id)]),
     case Id of

src/aeso_parser.erl

@@ -359,9 +359,12 @@ exprAtom() ->
         , ?RULE(tok('['), Expr, binop('..'), Expr, tok(']'), _3(_2, _4))
         , ?RULE(keyword('('), comma_sep(Expr), tok(')'), tuple_e(_1, _2))
         , letpat()
+        , hole()
         ])
     end).
 
+hole() -> ?RULE(token('???'), {id, get_ann(_1), "???"}).
+
 comprehension_exp() ->
     ?LAZY_P(choice(
       [ comprehension_bind()