[Haskell-cafe] PSA for Cabal 2.2 new-* users regarding .ghc.environment files

There is an important change in the cabal new- commands for 2.2 that the release docs should have highlighted more significantly. Cabal new-* commands now produce a .ghc.environment file by default. These files [1] are picked up by ghc and ghci automatically (since 8.0.1), and allow them to operate directly with the same package environment used by the new-* commands. This lets you, for example, run `ghci` in a project where you are using `new-build` and get the proper dependencies in scope. Herbert has written an experimental tool to make it easier to create and manipulate these environments [2]. However: there is a drawback (on which some discussion at [3] and [4]. In particular, there is not good information provided by ghc about when these files are picked up and used. So if you are admixing new-* commands and other commands for the time being, jumping between the two, or admixing ghc and ghcjs, etc., then you may run into unexpected behavior! [5] The simplest solution for now is to delete the local .ghc.environment file in those cases (i.e. where you're mixing commands and get unexpected behavior). A particular gotcha is that these files are picked up not just in the current directory but also in any parent directory. Cheers, Gershom [1] documented at https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/packages.html#package-environments [2] https://github.com/hvr/cabal-env [3] https://github.com/haskell/cabal/issues/4542 [4] https://ghc.haskell.org/trac/ghc/ticket/13753 [5] Error messages may be like ".cabal/store/ghc-8.0.2/package.db/package.cache: openBinaryFile: does not exist (No such file or directory)" or complaints about missing inplace dependencies.