Clean up; Add example

dust-lang/src/abstract_tree/async_block.rs

@@ -39,18 +39,14 @@ impl AbstractNode for AsyncBlock {
     }
 
     fn evaluate(self, _context: &Context, _: bool) -> Result<Option<Evaluation>, RuntimeError> {
-        let statement_count = self.statements.len();
         let final_result = Mutex::new(Ok(None));
-
-        self.statements
-            .into_par_iter()
-            .enumerate()
-            .find_map_any(
-                |(index, statement)| -> Option<Result<Option<Evaluation>, RuntimeError>> {
+        let statement_count = self.statements.len();
+        let error_option = self.statements.into_par_iter().enumerate().find_map_any(
+            |(index, statement)| -> Option<RuntimeError> {
                 let result = statement.evaluate(&_context, false);
 
-                    if result.is_err() {
-                        return Some(result);
+                if let Err(error) = result {
+                    return Some(error);
                 }
 
                 if index == statement_count - 1 {
@@ -60,8 +56,13 @@ impl AbstractNode for AsyncBlock {
 
                 None
             },
-            )
-            .unwrap_or(final_result.into_inner()?)
+        );
+
+        if let Some(error) = error_option {
+            Err(error)
+        } else {
+            final_result.into_inner()?
+        }
     }
 
     fn expected_type(&self, _context: &Context) -> Result<Option<Type>, ValidationError> {

examples/async_count.ds

@@ -0,0 +1,19 @@
+count_slowly = fn (multiplier: int) {
+	i = 0
+
+	while i < 10 {
+		sleep_time = i * multiplier;
+
+		thread.sleep(sleep_time)
+		io.write_line(i as str)
+
+		i += 1
+	}
+}
+
+async {
+	count_slowly(50)
+	count_slowly(100)
+	count_slowly(200)
+	count_slowly(250)
+}