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>

test/aeso_compiler_tests.erl

@@ -1182,6 +1182,16 @@ failing_contracts() ->
     , ?TYPE_ERROR(using_contract_as_namespace,
                   [<<?Pos(5,3)
                      "Cannot use undefined namespace F">>])
+    , ?TYPE_ERROR(hole_expression,
+                  [<<?Pos(5,13)
+                     "Found a hole of type `bool`">>,
+                   <<?Pos(6,17)
+                     "Found a hole of type `string`">>,
+                   <<?Pos(9,37)
+                     "Found a hole of type `(int) => int`">>,
+                   <<?Pos(13,20)
+                     "Found a hole of type `'a`">>
+                  ])
     ].
 
 validation_test_() ->

test/contracts/hole_expression.aes

@@ -0,0 +1,13 @@
+include "List.aes"
+
+contract C =
+    entrypoint f() =
+        let ??? = true
+        let v = ???
+        let q = v == "str"
+        let xs = [1, 2, 3, 4]
+        switch (List.first(List.map(???, xs)))
+            Some(x) => x + 1
+            None    => 0
+
+    function g() = ???