Stack won't resolve a 'hidden' dependency

Question

I am working on my first major Haskell application, and want to add mockery to create disposable test WAI threads. Importing mockery and running stack test resulted in the compiler error:

Failed to load interface for ‘Test.Mockery.Directory’
It is a member of the hidden package ‘mockery-0.3.5’.
Perhaps you need to add ‘mockery’ to the build-depends in your .cabal file.
Use -v to see a list of the files searched for.

So, I added mockery to my cabal file under test dependencies. However, when I run stack build or stack test mockery is automatically removed from the cabal file.

I have also tried listing mockery-0.3.5 under extra-deps in the stack.yaml file. This unsurprisingly didn't work, since mockery is part of my lts, and extra deps is for packages outside of lts.

How can I get stack to recognize that mockery should be included as a dependency to to project?

Here is my stack.yaml:

flags: {}
ghc-options:
  ! '*': -Wall
packages:
- .
extra-deps: [
]

resolver: lts-9.5

I'm using stack version 1.5.1

I imagine this is a stupid build issue and look forward to confronting my obvious oversight.

Possible duplicate of stack.yaml file & .cabal file differences? — Alexis King
Actually, no, that’s probably not a duplicate, reading your question more closely. It sounds like you are using hpack, which generates the .cabal file for you. Add the dependency in the package.yaml file, instead. — Alexis King

MCH MCH · Accepted Answer · 2019-11-18T23:16:05

In stack.yaml you declare the Stackage LTS version, a curated list of hackage dependencies that you want to depend on. You can also depend on local packages and packages in git that are not in Hackage. You may also change the versions of the packages in LTS as long as they respect the constraints of the other dependencies.

package.yaml is the build file. Any packages you want to import directly in your Haskell code must be declared in here as dependencies, even if they are explicitly declared in the stack.yaml.

Finally, when you see It is a member of the hidden package, that means that one of your dependencies is using that package, but it is not declared as a dependency in your build file.

Stack won't resolve a 'hidden' dependency

1 Answers