Description
Ormolu has very slow to build dependencies (essentially ghc-lib-parser). This has been an obstacle to contributing, and makes the CI very slow on the first push of a new branch.
Currently, the ./format script build Ormolu itself, via stack build. This is convenient (calling ./format is machine independent). The main reason why we went this route back then, was that the version of Ormolu that was available on Nixpkgs was too old, we needed a recent one. Now, I believe that it should be easy to provide Ormolu via Nixpkgs.
This doesn't prevent contributors who want to provide Ormolu via Stack to call stack build ormolu && stack exec -- ./format. It makes the management of dependencies more package-manager agnostic. And will let us download cached Ormolu when using Nix (in particular on CI).