How can I generate a coverage overlay with HPC and Stack?

Question

I have a small/medium sized Haskell codebase for which I'd like to generate a coverage report. By default the coverage report that HPC provides is filled with false negatives (e.g. I use lenses to access most of my record fields, rather than the auto generated ones, which HPC then reports as not covered).

The typical solution to this is to generate some overlay as specified here: https://wiki.haskell.org/Haskell_program_coverage#Hpc_toolkit

When I try to do this with Stack I run into an issue.

$ stack new --resolver=lts-9.5 hpcTest

If we edit src/Lib.hs to be:

module Lib
( someFunc, otherFunc
) where

someFunc :: IO ()
someFunc = putStrLn "someFunc"

otherFunc :: IO ()
otherFunc = putStrLn "otherFunc"

and test/spec.hs to be:

import Lib

main :: IO ()
main = someFunc

and run stack test --coverage

We get a coverage report which is not 100% covered, so if we follow the instructions of the wiki page we get: stack exec hpc -- draft --hpcdir=.stack-work/dist/x86_64-linux/Cabal-1.24.2.0/hpc/ --srcdir=. .stack-work/install/x86_64-linux/lts-9.5/8.0.2/hpc/hpcTest/hpcTest-test/hpcTest-test.tix > myDraft.txt

which when we run results in the following in myDraft.txt:

module "hpcTest-0.1.0.0-HnYRxRg1qoiyMKwsCMtby:Lib" {
  tick function "otherFunc" on line 9;
}

when we then do the next step and try to generate the overlay stack exec hpc -- overlay --hpcdir=.stack-work/dist/x86_64-linux/Cabal-1.24.2.0/hpc/ --srcdir=. myDraft.txt the process falls over and we get the following error:

hpc: can not find hpcTest-0.1.0.0-HnYRxRg1qoiyMKwsCMtby:Lib in ./.hpc, ./.stack-work/dist/x86_64-linux/Cabal-1.24.2.0/hpc/
CallStack (from HasCallStack):
  error, called at libraries/hpc/Trace/Hpc/Mix.hs:122:15 in hpc-0.6.0.3:Trace.Hpc.Mix

What am I doing wrong in this process and how can I get this to work?

Answer 1

Replace : with /. The draft code shall be

module "hpcTest-0.1.0.0-HnYRxRg1qoiyMKwsCMtby/Lib" {
  tick function "otherFunc" on line 9;
}

I find that by looking into the tix file stack hpc creates. (lol)