Splint

Splint is a proof of concept, showing how to use HLint 3 as a GHC source plugin. It is similar to hlint-source-plugin by Ollie Charles, except that it doesn't have to re-parse the module in order to lint it.

To use Splint, pass -fplugin=Splint to GHC. Any ideas suggested by HLint will be reported as warnings by GHC. For example, if you define Main.hs as:

main = print . concat $ map pure [ 'a' .. 'z' ]

You would expect HLint to tell you to use concatMap. Normally you would need to both compile your module with GHC and lint it with HLint. However with Splint you can compile it and get suggestions from HLint all at once by running:

ghc -fplugin=Splint Main.hs

Among all the usual output from GHC, you should see this new warning:

Main.hs:1:8: warning:
    Use concatMap
    Perhaps: print (concatMap pure ['a' .. 'z'])
  |
1 | main = print . concat $ map pure [ 'a' .. 'z' ]
  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

And that's all there is to it! HLint suggestions as part of your normal build process. What a time to be alive.

If you want to pass arguments through to HLint, you can use -fplugin-opt=Splint:arg. For example you can ignore the warning above with -fplugin-opt=Splint:'--ignore=Use concatMap'.

Trade offs

Running HLint as a GHC source plugin has some upsides:

• Modules are only linted when they're compiled, so if they haven't changed they won't be linted again.

• HLint's suggestions are reported just like GHC's warnings. They're formatted the same way and they're reported at the same time.

• Each module is only parsed once.

• Parsing is done by GHC instead of something like haskell-src-exts. HLint already works like this, but by using a plugin you can be sure that all of the versions and options line up correctly.

However it's also got some downsides:

• Using Splint means adding it as a dependency to the targets you want to lint. Normally HLint is either a test dependency or just installed on the system.

  You may be able to lessen the impact of this by providing a flag to control linting. That way you can enable it locally and in CI, but not require everything downstream of you to depend on HLint.

  flag lint
    default: False
    manual: True
  library
    if flag(lint)
      build-depends: splint
      ghc-options: -fplugin=Splint
  

• It's slower. I've found that it adds about a tenth of a second per module.

• You can't use the automated refactorings that HLint provides.

• Using plugins marks every module as unsafe.

To do

As stated, Splint is basically a tech demo. Although it's usable in its current form, there is some low hanging fruit to fix before it should be considered ready for production:

• Work with versions of GHC older than 8.10.
• Accept command-line options using -fplugin-opt=Splint:something.
• Reliably read HLint configuration.
• Avoid re-reading HLint config for each source file.
• Figure out a good output format for the warnings.
• Publish to Hackage.