Improve nix install docs for projects that depend on dhall-haskell

[jmitchell]

Directions in the getting started wiki page say to install dhall-json like this:

nix-env --install --attr dhall-json

Running that in a local clone of dhall-lang/dhall-json resulted in the following error:

Setup: Encountered missing dependencies:
dhall >=1.9.0 && <1.10

Can the docs be improved to avoid this problem? It would help those who aren't yet familiar with nix and just want to get things working. The directions in various dhall-* projects should be reviewed too.

One potential solution is to add a symlink in ~/.nix-defexpr:

ln -s $DHALL_HASKELL_CLONE_DIR/release ~/.nix-defexpr/dhall

[jmitchell]

After trying to reproduce the issue from scratch I realize my current solution is actually a little more involved. I have the following symlinks in ~/.nix-defexpr:

dhall -> s/dhall-lang/dhall-haskell/release
dhall-bash -> s/dhall-lang/dhall-bash/release
dhall-json -> s/dhall-lang/dhall-json/release
dhall-nix -> s/dhall-lang/dhall-nix/release
dhall-text -> s/dhall-lang/dhall-text/release

And now I can install dhall-json by running nix-env -i dhall-json from any directory.

[Gabriella439]

What version of nixpkgs are you using?

[jmitchell]

I'm using release-17.09. I notice in master dhall's version is 1.8.2, which is outside the required version bounds.

I discovered at some point after the ~/.nix-defexpr hackery that dhall --version unexpectedly reported 1.6.0, the version from release-17.09. I can build and install dhall-haskell v1.9.1 using release.nix from its repo and get dhall --version to report 1.9.1. However, I actually don't have a user-friendly way to install the latest dhall-json yet.

[Gabriella439]

Ah, got it! Yeah, I can update the instructions to support installing the latest version

[Profpatsch]

I recommend against the usage of nix-env if possible, since it introduces state. Especially when trying to develop Haskell packages (which I’m assuming you did, because you had a local clone of dhall-json) you should follow the developer instructions in the nixpkgs manual.

[jmitchell]

For now I'm primarily interested in a supported workflow for installing the latest dhall* binaries using nix. My intent is to tinker with the utilities and custom dhall configs before doing any Haskell development. I suspect that's a common scenario for users.

[Profpatsch]

Then executing nix-shell -p dhall dhall-json dhall-nix dhall-text dhall-bash -I nixpkgs=/path/to/nixpkgs/checkout/on/recent/master/or/unstable should do what you want and throw you into a shell with all executables in the PATH.

[Gabriella439]

@jmitchell: I updated the release.nix for each project to specify a dhall-1.9.0 dependency, which should fix the problem that you ran into