by ndmitchell

ndmitchell / weeder

Detect dead exports or package imports

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Weeder Hackage version Stackage version Linux build status Windows build status

Weeder has moved!

Weeder 2.0 is being developed at https://github.com/ocharles/weeder on different foundations. This repo is for historical reference only.

Most projects accumulate code over time. Weeder detects unused Haskell exports, allowing dead code to be removed (pulling up the weeds). Weeder piggy-backs off files generated by

, so first obtain stack, then:

  • Install
    by running
    stack install weeder --resolver=nightly
  • Ensure your project has a
    file. If you don't normally build with
    then run
    stack init
    to generate one.
  • Make sure you have
    ghc-options: {"$locals": -ddump-to-file -ddump-hi}
    in your
  • Run
    weeder . --build
    , which builds your project with
    and reports any weeds.

What does Weeder detect?

Weeder detects a bunch of weeds, including:

  • You export a function
    from module
    , but nothing else in your package uses
    , and
    is not an
    . Therefore, the export of
    is a weed. Note that
    itself may or may not be a weed - once it is no longer exported
    will tell you if it is entirely redundant.
  • Your package
    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
    field that are either unused (and thus should be deleted), or are missing (and thus should be added). The
    tool warns about the latter already.
  • A source file is used between two different sections in a
    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
    file. This situation can be because the file is unused, or the
    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.

and the Cabal
), as these may mean that something is currently a weed, but in different configurations it is not.

I recommend fixing the warnings relating to

and files not being compiled first, as these may cause other warnings to disappear.

What else should I use?

Weeder detects dead exports, which can be deleted. To get the most code deleted from removing an export, use:

  • 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.
  • Unused, which works at the level of
    information, so can be used if you don't want to use
    , can't compile your code, or want to detect unused code between projects.

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

file adjacent to the
with a list of exclusions. To generate an initial list of exclusions run
weeder . --yaml > .weeder.yaml

You may wish to generalise/simplify the

by removing anything above or below the interesting part. As an example of the
file from
- 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

works, and I am aware of it). It also says that I am not concerned about
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

On the CI you should then run

weeder .
weeder . --build
to compile as well). To avoid the cost of compilation you may wish to fetch the latest Weeder binary release.

For the CI systems Travis, Appveyor and Azure Pipelines add the line:

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

The arguments after

are passed to
, so modify the final
if you want other arguments. This command works on Windows, Mac and Linux.

What about Cabal users?

Weeder requires the textual

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
files afterwards. I welcome patches to do that integration.

What about false positives?

Weeder strives to avoid incorrectly warning about something that is required, if you find such an instance please report it on the issue tracker. Unfortunately there are some cases where there are still false positives, as GHC doesn't put enough information in the


Data.Coerce If you use

the constructors for the data type must be in scope, but if they aren't used anywhere other than automatically by
then Weeder will report unused imports. You can ignore such warnings by adding
- message: Unused import
to your

Declaration QuasiQuotes If you use a declaration-level quasi-quote then weeder won't see the use of the quoting function, potentially leading to an unused import warning, and marking the quoting function as a weed. The only solution is to ignore the entries with a


Stack extra-deps Packages marked extra-deps in your

will be weeded, due to a bug in
. The only solution is to ignore the packages with a

Linking to C functions If a library provides C functions, and these are used directly from another library/executable, the library providing these functions may be marked as a redundant

, see more details.

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