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

[Dan Burton]

>
> 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! The simplest solution for now is to delete the local
> .ghc.environment file in those cases


Is `cabal exec` suitable for working around this issue?
If not, is there something like `cabal new-exec` planned for the future?

-- Dan Burton

On Sat, May 12, 2018 at 1:41 PM, Gershom B <gershomb at gmail.com> wrote:

> 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.
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