dhall-nix?

[MichaelXavier]

This may be more of a question than an issue. I noticed in the main README you mention that nix has worse error messages and that is owed to not having a type system. I'm wondering if you have used dhall-nix to address this. The syntax is so similar that it doesn't seem like it would add much overhead in terms of noise. Is the problem that you'd need to layer a build process (make?) over the whole project to convert your source-controlled dhall expressions into git-ignored nix expressions?

[Gabriella439]

@MichaelXavier: You don't need to add an additional build process. There is a dhallToNix function I added to Nixpkgs which will convert Dhall source code to Nix at evaluation time (using import-from-derivation):

https://github.com/NixOS/nixpkgs/blob/92a047a6c4d46a222e9c323ea85882d0a7a13af8/pkgs/build-support/dhall-to-nix.nix

Note that it's restricted to an older version of dhall until hnix publishes a new release.

That said, I haven't made a serious attempt to use dhall-nix to replace Nix yet. dhall-nix is so far mostly a "proof-of-concept" project.

[MichaelXavier]

Thanks for the quick response, @Gabriel439. I'm curious if you have any doubts about its usability as a replacement. Maybe when you get used to the nix syntax and error messages, that particular downside to the nix expression language isn't a big enough problem to bother with?

[Gabriella439]

@MichaelXavier: The main issue is that Nixpkgs depends heavily on weakly-typed idioms and general recursion which you can't embed in Dhall. So you can carve out some strongly-typed islands using Dhall, but you'll hit a wall pretty quickly. I think a slightly more promising approach is https://github.com/dhallix which is basically an attempt to redo something like Nixpkgs except in a way more idiomatic to Dhall, but that requires significant performance improvements to the interpreter to be realistic

[MichaelXavier]

@Gabriel439 thanks! All of that makes sense. I've got a few more hours under my belt with dhall and started to see where nix exceeds the functionality that dhall limits itself to or how performance could be an issue when you're evaluating huge expressions.

[Gabriella439]

You're welcome! 🙂

[midirhee12]

Note that it's restricted to an older version of dhall until hnix publishes a new release.

@Gabriel439
Is there any update on this? I love the dhall project to death.

[Gabriella439]

@cvoges12: This will be fixed in the next release (~ 1 week from now). See: dhall-lang/dhall-haskell#1000