7 ms·

I guess TDD can be great for parsers.

• jononor 9y ago
  In the classical way of setting up the parser, passing some example input and then asserting some things about the output? It's good for covering basic features/usecases. Since setup is always the same, it should be written as a list of input/expectation pairs (data driven). But coverage is typically limited by the effort needed to write the assertions - which grows in parallel with the input complexity. If one also writes a serializer, then one can additionally test for any input the property: `serialize(parse(input)) === input`. This means adding a new test is just dropping in more example inputs (say from bug reports). Now one can go further, and define a set of mutation operators that can act on input to produce a new valid input. Reorder tokens, change data at leaves, delete and insert new data, etc. Now one can generate arbitrary amounts of new test cases based on existing input examples. Other mutation operations can be designed to generate invalid inputs, which should always give an error (never crash, halt or unexpected exception).