feat(ast): handle the empty tuple type

Signed-off-by: ljedrz <ljedrz@gmail.com>

ast/src/leo.pest

@@ -226,7 +226,7 @@ dimension_single = {
 // Declared in types/array_dimensions.rs
 dimension_multiple = { "(" ~ number_positive ~ ("," ~ number_positive)* ~ ")"}
 
-type_tuple = { "(" ~ NEWLINE* ~ type_ ~ ("," ~ NEWLINE* ~ type_)+ ~ ","? ~ NEWLINE* ~ ")" }
+type_tuple = { "(" ~ NEWLINE* ~ (type_ ~ ("," ~ NEWLINE* ~ type_)+ ~ ","?)? ~ NEWLINE* ~ ")" }
 
 /// Values
 

ast/tests/tuple.rs

@@ -59,7 +59,7 @@ fn access() {
 }
 
 #[test]
-fn unit() {
+fn implicit_unit() {
     parses_to! {
         parser: LanguageParser,
         input:  "()",
@@ -69,3 +69,23 @@ fn unit() {
         ]
     }
 }
+
+#[test]
+fn explicit_unit() {
+    parses_to! {
+        parser: LanguageParser,
+        input:  "let x: () = ();",
+        rule:   Rule::statement_definition,
+        tokens: [
+            statement_definition(0, 15, [
+                declare(0, 4, [let_(0, 4, [])]),
+                variables(4, 9, [
+                    variable_name(4, 5, [identifier(4, 5, [])]),
+                    type_(7, 9, [type_tuple(7, 9, [])])
+                ]),
+                expression(12, 14, [expression_term(12, 14, [expression_tuple(12, 14, [])])]),
+                LINE_END(14, 15, [])
+            ])
+        ]
+    }
+}