VSCode HLint

Lint Haskell code with HLint in Visual Studio Code.

Prerequisites

• Stack must be in $PATH.
• hlint 2.0.8 or newer must be available in the current stack project. HLint 1.9.25 and upwards should work too, but I'm not testing it. HLint 2.0.0 to 2.0.7 do not work—these versions can't read code from standard input.

To add hlint to a stack project run stack build hlint (build, not install!) in the stack project.

To apply suggestions apply-refact 0.3 or newer must be available in the current stack project. If it's missing you'll see a warning message and the light bulb feature won't be available for HLint suggestions, but linting will continue to work just fine.

To add apply-refact to a stack project run stack build apply-refact in the top-level directory of the project.

Usage

Just open or save a Haskell file. HLint will automatically check your file.

In some cases HLint can automatically fix issues. In these cases a code action is available on the problematic expression; just click on the light bulb in the left margin.

Note: By default most HLint hints are "suggestions". VSCode doesn't show these in the editor; you'll need to summon the "Problems" window explicitly to see those. You can configure HLint to change the severity of hints if you like.

Prior Art

Haskell Linter.

I wrote this extension because the above does not currently work with HLint 2, and I found the implementation overly verbose and clumsy when trying to address the issue. If I may say this extension has the better code: Less mutable state, much simpler implementation, and better documentation :blush: It also uses refactor to apply HLint suggestions instead of text replacement.