Last updated: 2017-06-15 17:55:15 +0100

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Tests for runtime-arbitrary

This is necessarily a separate module to runtime-arbitrary as that package must be available to nix-eval for these tests to work. To run the tests, use cabal run.

These tests are necessary as runtime-arbitrary does not provide any static guarantees that getArbGen will provide any Arbitrary instances. In fact, by default it will return an empty list!

To make it produce anything at all, we must insert the following line into our program (or into the preamble of a nix-eval expression):

mkIfCxtInstances ''Arbitrary

This uses template haskell to look up all instances of the Arbitrary typeclass available at that point, and make them available to getArbGen. The placement of this line is important: it will only find instances from those modules which have been imported at that point. This is why we don't perform this step automatically in runtime-arbitrary: the available instances would be limited to what runtime-arbitrary has imported; by requiring users to insert this themselves, all of their imports will be available too.