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CONTRIBUTING.md

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Tests

ppx_yojson uses two layers of tests. Adding tests along with your contribution is definitely encouraged and appreciated!

Unit tests

You can find unit tests for simple library functions in test/lib. They are written using OUnit. There is one test file per tested library module and the entire test suite is defined and run in test_ppx_yojson_lib.ml.

We should try testing stuff there as much as possible, especially small helper functions that don't produce chunks of AST.

Each function has its own test suite under test_<function_name> and they are all grouped together under a suite value at the test module's top level.

We try to mimic the module structure of the project in the tests. That means that if you want to add tests for a function from a submodule B from a module A, it should sit under a Test_a.B module which should have its own suite.

Rewriter tests

These are the main tests for ppx_yojson. We use them to ensure the generated code is what we expect it to be.

They are run by comparing the output of ppx_yojson as a standalone binary against .expected files, both for successful and unsuccessful rewriting.

The successful cases are tested in test/rewriter/test.ml. You can update it with new test cases and then run dune runtest. It will show the diff with the generated code for your new case, if you're happy with the result you can update the expected results by running dune promote.

We also test rewriting errors to make sure they are properly triggered and located. There's one file per error case tested in test/rewriter/errors. If you want to add a test case there you should:

  1. run touch test/rewriter/errors/expr_<test_case_name>.{ml,expected} to add the new empty test cases. Use expr_ or pat_ as a prefix depending on which of the two extensions they correspond to.
  2. run dune runtest --auto-promote to update the dune file with the rules for your new test case.
  3. update the .ml file with the erronous use of ppx_yojson, you can use one of the other test cases as an example. If your error is already produced correctly, merlin should highlight it in your code.
  4. run dune runtest to see what the error is.
  5. if you're happy with the result, run dune promote to update the .expected file.