Announcing Weeder: dead export detection

Most projects accumulate code over time. To combat that, I've written Weeder which detects unused Haskell exports, allowing dead code to be removed (pulling up the weeds). When used in conjunction with GHC -fwarn-unused-binds -fwarn-unused-imports and HLint it will enable deleting unused definitions, imports and extensions.

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 reports any 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.

Running with Continuous Integration

Before running Weeder on your continuous integration (CI) server, you should first ensure there are no existing weeds. One way to achieve that is to ignore existing hints by running weeder . --yaml > .weeder.yaml and checking in the resulting .weeder.yaml.

On the CI you should then run weeder . (or weeder . --build to compile as well). To avoid the cost of compilation you may wish to fetch the latest Weeder binary release. For certain CI environments there are helper scripts to do that.

Travis: Execute the following command:

curl -sL https://raw.github.com/ndmitchell/weeder/master/misc/travis.sh | sh -s .

The arguments after -s are passed to weeder, so modify the final . if you want other arguments.

Appveyor: Add the following statement to .appveyor.yml:

- ps: Invoke-Command ([Scriptblock]::Create((Invoke-WebRequest 'https://raw.githubusercontent.com/ndmitchell/weeder/master/misc/appveyor.ps1').Content)) -ArgumentList @('.')

The arguments inside @() are passed to weeder, so add new arguments surrounded by ', space separated - e.g. @('.' '--build').

What about Cabal users?

Weeder requires the textual .hi file for each source file in the project. Stack generates that already, so it was easy to integrate in to. There's no reason that information couldn't be extracted by either passing flags to Cabal, or converting the .hi files afterwards. I welcome patches to do that integration.

Haskell Website Working Group - Update

Summary: We have agreed a set of principles for the website and are collecting information.

I'm writing this partly in my capacity as the chair of the Haskell Website Working Group, and partly as an individual (so blame me rather than the committee). It's fair to say that the original goal of the committee was to make sure everyone agrees on the download page. Discussions amongst the committee lead to a shared goal that the download page itself should clearly direct users along a precise path, without requiring beginners to make decisions requiring judgement. That probably means that download page should only describe one installation path, pushing alternatives onto a separate page.

To decide what should go on the download page, our first step was to evaluate what was currently available, and what experience a beginner might have. We've started that on this page. As an example, it says how to install Haskell, how to open ghci, how to install a tool etc - all using the different options.

When I actually tried installing and using the various options listed on the current download page, they all had confusing steps, unintuitive behaviour and problems that I had to overcome. As an example, Stack on Windows recommends using the 32bit version, while noting that only the 64bit version works. At the same time, Core Platform starts by telling me to edit a global Cabal config file.

I invite everyone to help contribute to that page, via pull requests. At the same time, it would be great if the issues raised could be ironed out, leading to a smooth beginner experience (I'm talking to maintainers in person and raising GitHub tickets). Once the information is collected, and ideally the tools have improved, it will be time to make a decision between the options. When the decision comes, it will be technically motivated, and hopefully unambiguous.