Generating coverage output for Haskell tests

[SebastianKG]

This is my work towards a code coverage solution for Haskell in Bazel. Running this is done best in this form:

bazel coverage //tests/two-libs:two-libs -s --test_output=all

This will show you the coverage report of the chosen haskell_test rule. This is not the standard Bazel coverage report, but instead a custom-made solution to get around the current restrictions for bazel coverage's report-collection capabilities.

However, the above won't do anything on its own. To get a coverage report for a haskell_test, you must establish a required base level of coverage, like this:

haskell_test(
    name = "two-libs",
    srcs = ["Main.hs"],
    expected_expression_coverage = 80,
    deps = [
        ":two",
        "//tests/hackage:base",
    ],
)

This will require expression coverage >= 80%. Only expression coverage is currently supported, for simplicity.

I have noted that not all of the test rules pass yet with this new functionality, but I don't know if immediately handling complex cases is the best approach. My proposal is that we incrementally improve the flexibility of Haskell code coverage in Bazel. After all, it has worked for 0% of the test cases until now. two-libs is an example of a test case which works.

[guibou]

LGTM.

[Profpatsch]

Am going to review tomorrow.

[Profpatsch]

Looking good! A few trivial and one quoting/templating improvement.

All changes have been made.