diff --git a/src/ast.rs b/src/ast.rs
index 13127fe4..06d3b714 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -823,7 +823,7 @@ pub enum Stmt {
         Position,
     ),
     /// `while` expr `{` stmt `}`
-    While(Expr, Box<Stmt>, Position),
+    While(Option<Expr>, Box<Stmt>, Position),
     /// `do` `{` stmt `}` `while`|`until` expr
     Do(Box<Stmt>, Expr, bool, Position),
     /// `for` id `in` expr `{` stmt `}`
@@ -981,9 +981,10 @@ impl Stmt {
                     && x.0.values().all(Stmt::is_pure)
                     && x.1.as_ref().map(Stmt::is_pure).unwrap_or(true)
             }
-            Self::While(condition, block, _) | Self::Do(block, condition, _, _) => {
+            Self::While(Some(condition), block, _) | Self::Do(block, condition, _, _) => {
                 condition.is_pure() && block.is_pure()
             }
+            Self::While(None, block, _) => block.is_pure(),
             Self::For(iterable, x, _) => iterable.is_pure() && x.1.is_pure(),
             Self::Let(_, _, _, _) | Self::Const(_, _, _, _) | Self::Assignment(_, _) => false,
             Self::Block(block, _) => block.iter().all(|stmt| stmt.is_pure()),
@@ -1021,10 +1022,11 @@ impl Stmt {
                     s.walk(path, on_node);
                 }
             }
-            Self::While(e, s, _) | Self::Do(s, e, _, _) => {
+            Self::While(Some(e), s, _) | Self::Do(s, e, _, _) => {
                 e.walk(path, on_node);
                 s.walk(path, on_node);
             }
+            Self::While(None, s, _) => s.walk(path, on_node),
             Self::For(e, x, _) => {
                 e.walk(path, on_node);
                 x.1.walk(path, on_node);
diff --git a/src/engine.rs b/src/engine.rs
index 8c710c69..113150f9 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -2138,24 +2138,25 @@ impl Engine {
 
             // While loop
             Stmt::While(expr, body, _) => loop {
-                match self
-                    .eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
-                    .as_bool()
-                {
-                    Ok(true) => {
-                        match self.eval_stmt(scope, mods, state, lib, this_ptr, body, level) {
-                            Ok(_) => (),
-                            Err(err) => match *err {
-                                EvalAltResult::LoopBreak(false, _) => (),
-                                EvalAltResult::LoopBreak(true, _) => return Ok(Dynamic::UNIT),
-                                _ => return Err(err),
-                            },
-                        }
-                    }
-                    Ok(false) => return Ok(Dynamic::UNIT),
-                    Err(err) => {
-                        return Err(self.make_type_mismatch_err::<bool>(err, expr.position()))
+                let condition = if let Some(expr) = expr {
+                    self.eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
+                        .as_bool()
+                        .map_err(|err| self.make_type_mismatch_err::<bool>(err, expr.position()))?
+                } else {
+                    true
+                };
+
+                if condition {
+                    match self.eval_stmt(scope, mods, state, lib, this_ptr, body, level) {
+                        Ok(_) => (),
+                        Err(err) => match *err {
+                            EvalAltResult::LoopBreak(false, _) => (),
+                            EvalAltResult::LoopBreak(true, _) => return Ok(Dynamic::UNIT),
+                            _ => return Err(err),
+                        },
                     }
+                } else {
+                    return Ok(Dynamic::UNIT);
                 }
             },
 
@@ -2170,15 +2171,17 @@ impl Engine {
                     },
                 }
 
-                match self
+                if self
                     .eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
                     .as_bool()
+                    .map_err(|err| self.make_type_mismatch_err::<bool>(err, expr.position()))?
                 {
-                    Ok(true) if !*is_while => return Ok(Dynamic::UNIT),
-                    Ok(false) if *is_while => return Ok(Dynamic::UNIT),
-                    Ok(_) => (),
-                    Err(err) => {
-                        return Err(self.make_type_mismatch_err::<bool>(err, expr.position()))
+                    if !*is_while {
+                        return Ok(Dynamic::UNIT);
+                    }
+                } else {
+                    if *is_while {
+                        return Ok(Dynamic::UNIT);
                     }
                 }
             },
diff --git a/src/optimize.rs b/src/optimize.rs
index cff6580b..2f367a05 100644
--- a/src/optimize.rs
+++ b/src/optimize.rs
@@ -393,25 +393,32 @@ fn optimize_stmt(stmt: &mut Stmt, state: &mut State, preserve_result: bool) {
         }
 
         // while false { block } -> Noop
-        Stmt::While(Expr::BoolConstant(false, pos), _, _) => {
+        Stmt::While(Some(Expr::BoolConstant(false, pos)), _, _) => {
             state.set_dirty();
             *stmt = Stmt::Noop(*pos)
         }
         // while expr { block }
         Stmt::While(condition, block, _) => {
             optimize_stmt(block, state, false);
-            optimize_expr(condition, state);
+
+            if let Some(condition) = condition {
+                optimize_expr(condition, state);
+            }
 
             match **block {
                 // while expr { break; } -> { expr; }
                 Stmt::Break(pos) => {
                     // Only a single break statement - turn into running the guard expression once
                     state.set_dirty();
-                    let mut statements = vec![Stmt::Expr(mem::take(condition))];
-                    if preserve_result {
-                        statements.push(Stmt::Noop(pos))
-                    }
-                    *stmt = Stmt::Block(statements, pos);
+                    if let Some(condition) = condition {
+                        let mut statements = vec![Stmt::Expr(mem::take(condition))];
+                        if preserve_result {
+                            statements.push(Stmt::Noop(pos))
+                        }
+                        *stmt = Stmt::Block(statements, pos);
+                    } else {
+                        *stmt = Stmt::Noop(pos);
+                    };
                 }
                 _ => (),
             }
diff --git a/src/parser.rs b/src/parser.rs
index 96e1da4e..cb549a41 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -2122,9 +2122,10 @@ fn parse_while_loop(
     let (guard, token_pos) = match input.next().unwrap() {
         (Token::While, pos) => {
             ensure_not_statement_expr(input, "a boolean")?;
-            (parse_expr(input, state, lib, settings.level_up())?, pos)
+            let expr = parse_expr(input, state, lib, settings.level_up())?;
+            (Some(expr), pos)
         }
-        (Token::Loop, pos) => (Expr::BoolConstant(true, pos), pos),
+        (Token::Loop, pos) => (None, pos),
         (t, _) => unreachable!("expecting Token::While or Token::Loop, but gets {:?}", t),
     };
     settings.pos = token_pos;