YAML parsers usually do a good job in recognizing the correct types you are using. Parsing following YAML with for instance SnakeYAML will result in a Map containing the String “localhost” and the Integer 8080:
This is also working with mixed types in an Array:
config: [localhost, 8080]
Here we’ll get a List with a String and an Integer. While this is great for this case it may be not great for all cases. A common pitfall is to unintentionally mix types:
hexChars: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F]
Here we’ll get a List containing ten Integers and six Strings. Maybe not what we expected. While this example is quite obvious it may not always be so clear. I recently had problems with the list of Country-Codes defined in ISO-3166:
countryCodes: [AW, AF, AO, AI, AX, AL, AD, AE, AR, AM, AS, ...]
All of the entries are just two alphabetic characters so what could go wrong? It turns out that the Country-Code for Norway ‘NO’ is a boolean value for YAML.
Lessons learned: Always quote Strings in YAML. Always.