In general, stack is pretty good at caching build artifacts - the first build might be glacial, but incremental builds are snappy. This all goes out the window when you start playing with flags like –profile, though - stack sees that something has changed and dutifully rebuilds every-bloody-thing. This is a huge disincentive to casually run profiling builds.
thankfully, stack keeps all its bits and bobs inside ./.stack-work, so all you need to do is create a shadow directory like so:
mkdir foo-prof cd foo-prof lndir ../foo cd foo-prof rm -rf .stack-work stack build --profile
(on ubuntu, you can get lndir from xutils-dev.)
now, you have a directory with a separate .stack-work, and can mess around in that directory for all your profiling needs without slowing everything else down to a shuddering halt.
(nb: this will not cope terribly gracefully with new files being added. a workaround is to only symlink the top level files and directory instead of a full shadow tree: that way, only top level changes will break things.)
(another possibility would be to set STACK_WORK=.stack-work-profiling whenever you run profiling commands, but you’re going to screw it up eventually - probably better to have totally separate implicit contexts.)