The Setup.hs file is a component of the Cabal build system which stack uses. It’s technically not needed by stack, but it is still considered good practice in the Haskell world to include it. The file we’re using is straight boilerplate:

It is also important to remember that stack is built on top of the Cabal build system. Therefore, an understanding of the moving parts in Cabal are necessary. In Cabal, we have individual packages, each of which contains a single .cabal file. The .cabal file can define 1 or more components: a library, executables, test suites, and benchmarks. It also specifies additional information such as library dependencies, default language pragmas, and so on.