Added cast error and custom error to error_message

Author: Tko1

src/error_message.rs

@@ -49,3 +49,36 @@ pub fn generic_err(error: Box<dyn Error>) -> Value {
 pub fn unknown_err(error: String) -> Value {
     Value::Condition(error)
 }
+//
+// This module will likely be rewritten to look like everything below this line
+// The general idea is that for any submessage in an error that is truly 'variable', like
+// one that's sometimes:
+//
+// "Type mismatch: Expected an [Int]"
+//
+// And sometimes
+//
+// "Type mismatch: Expected a [Float that is greater than 12 but sometimes 5]"
+//
+// we cannot express this cleanly with just one pre-existing type, we need to create a new custom type
+// (or, if we wish to be a bit more flexible and basically forgo the type system, we can use a plain string).
+// In my case, I think really expressing either as either is fine, as long as the same error produces
+// the same message each time, which can be enforced with functions; ie
+// 
+// pub weird_error() -> String {
+//    "This always returns the same weird error"
+// }
+//
+// I don't forsee this the sort of thing for a bug to hide in, and one that requires strong type guarantees
+//
+
+// We currently don't have any type that represents an interface type name, so we
+pub struct Cast<'a>(pub &'a str);
+pub fn cast(expected: Cast,found: TypeTag) -> Value {
+    Value::Condition(format!("Cannot cast {} to {}",found,expected.0))
+}
+
+/// For one off errors
+pub fn custom(message: &str) -> Value {
+    Value::Condition(String::from(message))
+}