Multiple Cabal components

Currently, multiple Cabal components don’t work. You can work around this with the Cabal test-dev trick as described by the venerable Jade Lovelace. This works by defining a new component in our .cabal file which includes the sources from the library and the tests, which has the added benefit of speeding up compile times by allowing the compilation of different components to be interleaved.

You can see this demonstrated in the ghciwatch test sources here. We define four components:

• library
• tests
• An internal test-lib library
• An internal test-dev library

Then, we can use a command like cabal v2-repl test-dev to run a GHCi session containing both the library and test sources.

The package.yaml should look something like this:

---
spec-version: 0.36.0
name: my-simple-package
version: 0.1.0.0

flags:
  local-dev:
    description: Turn on development settings, like auto-reload templates.
    manual: true
    default: false

library:
  source-dirs: src

tests:
  test:
    main: Main.hs
    source-dirs:
      - test-main
    ghc-options: -threaded -rtsopts -with-rtsopts=-N
    when:
    - condition: flag(local-dev)
      then:
        dependencies:
        - test-dev
      else:
        dependencies:
        - my-simple-package
        - test-lib

internal-libraries:
  test-lib:
    source-dirs:
      - test

  test-dev:
    source-dirs:
      - test
      - src
    when:
    - condition: flag(local-dev)
      then:
        buildable: true
      else:
        buildable: false

Then, we can set the local-dev flag in our cabal.project.local, so that we use the test-dev target locally:

package my-simple-package
  flags: +local-dev

haskell-language-server

Defining the test-dev component does tend to confuse haskell-language-server, as a single file is now in multiple components. Fix this by writing an hie.yaml like this:

cradle:
  cabal:
    component: test-dev