Configuring Cabal build flags

It’s always an emergency when I go looking for this information!

Suppose you are building HLint.

mkdir ~/tmp && cd ~/tmp
curl -o hlint-3.2.7.tar.gz https://hackage.haskell.org/package/hlint-3.2.7/hlint-3.2.7.tar.gz
gunzip  hlint-3.2.7.tar.gz && tar xvf  hlint-3.2.7.tar

Left to their own devices, HLint and ghc-lib-parser-ex will default to auto mode meaning, they will decide for themselves if they should depend on ghc-lib-parser or native ghc libraries.

Sometimes it’s desirable to force the situation though and explicitly make them do one or the other. How you do that? The answer is of course package Cabal flags. There are two scenarios: building with stack or building with cabal.

• stack.yaml

  • Force link with ghc-lib-parser

      flags:
        hlint:
          ghc-lib: true
        ghc-lib-parser-ex:
          auto: false
          no-ghc-lib: false

  • Force link with native ghc

      flags:
        hlint:
          ghc-lib: false
        ghc-lib-parser-ex:
          auto: false
          no-ghc-lib: true

• cabal.project

  • Force link with ghc-lib-parser

      packages: hlint-3.2.7
      package hlint
        flags: +ghc-lib
      package ghc-lib-parser-ex
        flags: -auto -no-ghc-lib

  • Force link with native ghc

      packages: hlint-3.2.7
      package hlint
        flags: -ghc-lib
      package ghc-lib-parser-ex
        flags: -auto +no-ghc-lib

When working with Cabal in the HLint repository one can configure with command line constraints like this:

cabal new-build --constraint "hlint -ghc-lib" --constraint "ghc-lib-parser-ex -auto +no-ghc-lib"