CODEWITHSUNDEEP
haskellsmallcheck
Gspia
Gspia

Reputation: 809

testing (tasty-smallcheck) and file organisation and avoiding orphan instances

A library is in src-directory and there is a data definition A. Code related to testing is in test-directory, as suggested in many tutorials, and that includes Serial instance to generate the test cases, see links below:

instance Monad m => Serial m A where ...

cabal test warns that this is an orphan instance. Is this unavoidable or is there an established convention to organize libs, e.g., move serial instance definition to where data A is defined? The latter one somehow feels a bit wrong as it would be nice to keep testing related stuff at test-directory. If this is typical way, in what ways one can avoid exposing the testing related stuff at the public interface? Write 'internal' version of all tested modules (that the public ones and the actual testing program would use)? Something similar was suggested in unit-testing question.

Or does this matter at all? How bad convention it is to leave orphan instances in testing related code?

SmallCheck: Making types instance of typeclass Serial

How to use SmallCheck in Haskell?

Upvotes: 1

Views: 93

Answers (1)

hao
hao

Reputation: 10238

How bad convention it is to leave orphan instances in testing related code?

Not very. I agree with @Potonnier here. Orphan instances are unpleasant due to Haskell's open-world assumption, but I would argue that your testing executable target is a fairly closed world. We can reasonably expect that nobody will import the modules provided by the test executable package, and that the modules there are the "leaves" of their dependency graph. As long as that much is true, it is hard to imagine a scenario where orphan instances would lead to the worst-case scenario: something like a false positive, where the test passes due to an orphan instance when in fact it should have failed. It is not a binary decision, but I think the cost-benefit analysis here weighs against including the testing libraries as dependencies.

Upvotes: 1

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