@Override
    public StringType expectStringType() throws InnerParseError {
        throw new InnerParseError(String.format("Expected string but found %s. hint: use `assign` in a condition "
                + "or `isSet` to prove that this value is non-null", this));
    }
Gets the optional's contained value.

@return the optional's value.
smithy-rules-engine/src/main/java/software/amazon/smithy/rulesengine/language/evaluation/type/OptionalType.java