-fhpc assumes original sources relative to the current directory
For the most part, ghc can be invoked in a manner that is independent of the current directory by suitable use of
-fhpc it appears that the hpc bits always look in the current directory to find original source files.
Consider the following file layout:
Codec/Compression/Zlib/Stream.hsc dist/build/Codec/Compression/Zlib/Stream.hs examples/gunzip.hs
Now if we
examples and run:
ghc --make gunzip.hs -lz -i../ -i../dist/build
then it works fine. It finds
../dist/build and it never bothers to look for the original source of course.
-fhpc mode it has to also find the original sources so that it can match things back to them. However just adding -fhpc to the above command fails:
ghc --make gunzip.hs -lz -i../ -i../dist/build -fhpc [1 of 4] Compiling Codec.Compression.Zlib.Stream ( ../dist/build/Codec/Compression/Zlib/Stream.hs, ../dist/build/Codec/Compression/Zlib/Stream.o ) Codec/Compression/Zlib/Stream.hsc: getModificationTime: does not exist (No such file or directory)
So it's clearly not looking in the directory given by
-i.. for the original sources. Also, the failure mode is a bit unfriendly.
Cabal sometimes takes advantage of the fact that ghc can be invoked in a manner that is independent of the current directory, and we would like to add hpc support into Cabal.