Consider providing a unified report for "test --coverage"

[mgsloan]

The hpc program supports merging two tix files. This means that we can combine together the results of different test runs into a single coverage report.

Lets say package A uses package B, and package B's test suite doesn't have full coverage of its code. If package A's test suite ends up exercising these code paths in B, then this will show up in its individual coverage report. However, we won't see this coverage in B's report (and we wouldn't want to). If we can generate a unified coverage report for the whole stack project, then we can see this coverage without looking at all the individual reports.

When you have a lot of interdependent packages with test suites, this saves a bunch of laborious cross-checking of coverage reports.

Unfortunately, hpc merge does not work ideally for the use case of merging results from different binaries. In particular, I get hpc: mistmatched in module Main. Apparently .tix and .mix files do not disambiguate between modules when they're built into executables rather than libraries (which do get disambiguated). This causes other issues, such as described here.

All is not lost. What this means is that we need to exclude the modules which are built into the test binaries. For my test case, adding --exclude=Main does the trick. One hacky approach would be to parse out the error message from the hpc program and add exclude= flags as needed :)

[mgsloan]

Done!