Introduce length functor (#314)

air/src/execution_step/errors/catchable_errors.rs

@@ -91,6 +91,10 @@ pub enum CatchableError {
     /// Canon instruction can't canonicalize a stream since it's been found.
     #[error("stream with name {0} wasn't defined, so canon instruction can't canonicalize it")]
     StreamsForCanonNotFound(String),
+
+    /// This error type is occurred when the length functor applied to a value of non-array type.
+    #[error("the length functor could applied only to an array-like value, but it's applied to '{0}'")]
+    LengthFunctorAppliedToNotArray(JValue),
 }
 
 impl From<LambdaError> for Rc<CatchableError> {