Document how to specify build flags for dependencies

[ysangkok]

I tried - secp256k1-haskell:+ecdh >= 0.1.5 but it didn't work. The flag to be set is ecdh. I used this syntax because it was mentioned in https://docs.haskellstack.org/en/latest/GUIDE/#cabal-flag-management that you use minus to remove a flag. So I assumed that enabling a flag would be +.

The error from hpack is:

/home/travis/build/centromere/cacophony/package.yaml: Error while parsing $.library.dependencies[10] - invalid dependency "secp256k1-haskell:+ecdh >= 0.1.5"

[sol]

Hey! Sorry for the late reply and thanks for using hpack!

I think this is a limitation that we can't address in hpack.

.cabal files (and as a consequence package.yaml) does not support flags for dependencies.

However, stack allows you to specify flags for dependencies in stack.yaml. Here is a related stack overflow question: https://stackoverflow.com/questions/23523869/is-there-any-way-to-define-flags-for-cabal-dependencies

I hope this helps.

[sol]

I'm closing this issue. If you still have questions regarding stack and flags, please consult the relevant stack support channels.

For anything related to hpack, feel free to open a new issue in the future.