lts 9.9 is associated with ghc 8.0.2. why does it use 8.0.1

Question

I have a stack yaml with resolver lts-9.9 which is associated with ghc 8.0.2 (see https://www.stackage.org/lts-9.9 but when I run stack solver I see:

Using resolver: lts-9.9
Using compiler: ghc-8.0.1

and not ghc-8.0.2. This seems to violate that a stack build is constrained to exactly one set of packages and ghc? What do I overlook?

Answer 1

This can happen if ghc-8.0.1 is a system-based installed version of ghc, and stack is configured to use it instead of its own sandboxed version.

There is a pretty comprehensive page describing all the cases - https://docs.haskellstack.org/en/stable/faq/