Weeder

The principle is to delete dead code (pulling up the weeds). To do that, run:

• GHC with -fwarn-unused-binds -fwarn-unused-imports, which finds unused definitions and unused imports in a module.
• HLint, looking for "Redundant extension" hints, which finds unused extensions.
• The weeder tool, which detects functions that are exported internally but not available from outside this package. It also detects redundancies and missing information in your .cabal file.

Running Weeder

Weeder piggy-backs off files generated by stack, so first obtain stack, then:

• Install weeder by running stack install weeder --resolver=nightly.
• Ensure your project has a stack.yaml file. If you don't normally build with stack then run stack init to generate one.
• Run weeder . --build, which builds your project with stack and checks it for weeds.

What does Weeder detect?

Weeder detects a bunch of weeds, including:

• You export a function helper from module Foo.Bar, but nothing else in your package uses helper, and Foo.Bar is not an exposed-module. Therefore, the export of helper is a weed. Note that helper itself may or may not be a weed - once it is no longer exported -fwarn-unused-binds will tell you if it is entirely redundant.
• Your package depends on another package but doesn't use anything from it - the dependency should usually be deleted. This functionality is quite like packunused, but implemented quite differently.
• Your package has entries in the other-modules field that are either unused (and thus should be deleted), or are missing (and thus should be added). The stack tool warns about the latter already.
• A source file is used between two different sections in a .cabal file - e.g. in both the library and the executable. Usually it's better to arrange for the executable to depend on the library, but sometimes that would unnecessarily pollute the interface. Useful to be aware of, and sometimes worth fixing, but not always.
• A file has not been compiled despite being mentioned in the .cabal file. This situation can be because the file is unused, or the stack compilation was incomplete. I recommend compiling both benchmarks and tests to avoid this warning where possible - running weeder . --build will use a suitable command line.

Beware of conditional compilation (e.g. CPP and the Cabal flag mechanism), as these may mean that something is currently a weed, but in different configurations it is not.

I recommend fixing the warnings relating to other-modules and files not being compiled first, as these may cause other warnings to disappear.

Ignoring weeds

If you want your package to be detected as "weed free", but it has some weeds you know about but don't consider important, you can add a .weeder.yaml file adjacent to the stack.yaml with a list of exclusions. To generate an initial list of exclusions run weeder . --yaml > .weeder.yaml.

You may wish to generalise/simplify the .weeder.yaml by removing anything above or below the interesting part. As an example of the .weeder.yaml file from ghcid:

- message: Module reused between components
- message:
  - name: Weeds exported
  - identifier: withWaiterPoll

This configuration declares that I am not interested in the message about modules being reused between components (that's the way ghcid works, and I am aware of it). It also says that I am not concerned about withWaiterPoll being a weed - it's a simplified method of file change detection I use for debugging, so even though it's dead now, I sometimes do switch to it.