Add a Nix flake

[gilligan]

Summary
Add a basic nix flake that simply exports several packages from the toplevel default.nix

Notes

• This is mostly based on the work by @hamishmack in [WIP] Add flake #2517
• The flake is only added because using bitte requires it
• The current flake outputs are only temporary but in general the idea is for the flake to only provide what is essential for using bitte
• For the most part the flake just imports from niv sources and default.nix, but inputs.nixpkgs and inputs.haskell-nix have to be kept in sync with the niv revisions manually (for now?).

Status Quo

The latest state of this PR provides a flake.nix that "works" but:

• Outputs for the same package will differ between flake and non-flake builds
• This is because when importing haskell.nix as a flake there is no way to specify the revisions of hackage.nix and stackage.nix
• Fixing this is tracked in No way to specify hackage.nix/stackage.nix revisions via flake.nix input-output-hk/haskell.nix#1132
• @michaelpj and I agreed that we would merge this as-is because this is something that needs to be fixed upstream and we want to move on towards a bitte integration

---

Pre-submit checklist:

• Branch
  • Commit sequence broadly makes sense
  • Key commits have useful messages
  • Relevant tickets are mentioned in commit messages
  • Formatting, materialized Nix files, PNG optimization, etc. are updated
• PR
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested

Pre-merge checklist:

• Someone approved it
• Commits have useful messages
• Review clarifications made it into the code
• History is moderately tidy; or going to squash-merge

[michaelpj]

For the most part the flake just imports from niv sources and default.nix, but inputs.nixpkgs and inputs.haskell-nix have to be kept in sync with the niv revisions manually (for now?).

Good times...

[gilligan]

After chatting with @michaelpj we came to the conclusion that it should be possible to make this much simpler. So i tried, and failed 😅

In 1f1d22f I tried to just have the flake.nix import the default.nix directly by:

• making sure to pass system when importing sources.nix which is enough to actually use niv sources from flakes
• trying to use the correct haskell.nix overlay:

haskellNixOverlays =
if useFlakes 
  then [ haskellNix.allOverlays.combined-eval-on-build ]
  else haskellNix.overlays;

This failed in haskell.nix because it doesn't pass system when importing niv's sources.nix so I tried to fix that as well in
tweag/haskell.nix@3bf3616 and make sure to actually pass system when importing haskell.nix

But ultimately this still fails:

error: attribute 'currentSystem' missing                                                                                                                       
                                                                                                                                                               
       at /nix/store/g9vf0rrmn3g50g7fsn19vk0md0f4fya1-haskell.nix-src/nix/sources.nix:163:16:                                                                  
                                                                                                                                                               
          162|     , sources ? if isNull sourcesFile then {} else builtins.fromJSON (builtins.readFile sourcesFile)                                            
          163|     , system ? builtins.currentSystem                                                                                                           
             |                ^                                                                                                                                
          164|     , pkgs ? mkPkgs sources system                                                                                                              
                                                                                                                                                               
       … while evaluating the attribute 'system'                                                                                                               
                                                                                                                                                               
       at /nix/store/g9vf0rrmn3g50g7fsn19vk0md0f4fya1-haskell.nix-src/nix/sources.nix:65:91:                                                                   
                                                                                                                                                               
           64|       sourcesNixpkgs =                                                                                                                          
           65|         import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) { inherit system; };                                           
             |                                                                                           ^                                                     
           66|       hasNixpkgsPath = builtins.any (x: x.prefix == "nixpkgs") builtins.nixPath;                                                                
                                                                                                                                                               
       … while evaluating 'addContextFrom'

And i honestly don't understand why. Could you perhaps take a look @angerman or @hamishmack ? :) The key point here is that we are only adding the flake because we have to in order to be able to use bitte but we don't want to maintain or use it much otherwise. So ideally it would mostly just import stuff from the existing setup.

[gilligan]

@michaelpj as much as I had hoped that we might be able to make the flake.nix more non-intrusive i have to conclude that the current state of this PR is "as good as it gets". TBH though, while the alternative would have been nice(r) it's not that bad.

[gilligan]

While the latest sate of this builds, flake builds yield different outputs and i don't understand why that might be:

$ nix-build -A web-ghc -o ./web-ghc
$ nix build .#web-ghc-server
$ nix-diff $(nix-store --query --deriver ./result) $(nix-store --query --deriver ./web-ghc)

- /nix/store/znfwxgphsq7vj47c1gr34cq75m3zaprr-web-ghc-exe-web-ghc-server-0.1.0.0.drv:{out}
+ /nix/store/6xrgr181hh2fidr6v2amkw6j3m97c5pl-web-ghc.drv:{out}
• The set of input sources do not match:
    - /nix/store/y4wpxza56pmnsmknfjhazihwv69bdm82-plutus-root-web-ghc-exe-web-ghc-server-root
• The set of input names do not match:
    - data-default-class-lib-data-default-class-0.1.2.0
    - default-Setup-setup
    - exceptions-lib-exceptions-0.10.4
    - glibc-locales-2.32-24
    - http-types-lib-http-types-0.12.3
    - monad-logger-lib-monad-logger-0.3.36
    - optparse-applicative-lib-optparse-applicative-0.16.1.0
    - playground-common-lib-playground-common-0.1.0.0
    - prometheus-lib-prometheus-2.2.2
    - remove-references-to
    - servant-lib-servant-0.18.2
    - servant-server-lib-servant-server-0.18.2
    - wai-cors-lib-wai-cors-0.2.7
    - wai-extra-lib-wai-extra-3.1.6
    - wai-lib-wai-3.2.3
    - warp-lib-warp-3.3.14
    - web-ghc-exe-web-ghc-server-0.1.0.0-config
    - web-ghc-exe-web-ghc-server-0.1.0.0-ghc-8.10.4.20210212-env
    - web-ghc-lib-web-ghc-0.1.0.0
    - yaml-lib-yaml-0.11.5.0
    + ghc-shell-for-packages-ghc-8.10.4.20210212-env
    + hook
    + web-ghc-exe-web-ghc-server-0.1.0.0
• The input named `stdenv-linux` differs
  - /nix/store/5galpnsdfi02x7x0d3gn162k9jfsn4xr-stdenv-linux.drv:{out}
  + /nix/store/v6aj0hzvj7p9m4iiaqjs4d0nbrspc98f-stdenv-linux.drv:{out}
  • The environments do not match:
      allowedRequisites=''
          /nix/store/782n0xrdgdl1a32nvpdfnrx06xxa1zlp-gzip-1.10 /nix/store/782n0xrdgdl1a32nvpdfnrx06xxa1zlp-gzip-1.10 /nix/store/c5jgwpwxc1nxl9gzd9m4wlalrw7sr0z7-bzip2-1.0.6.0.1-bin /nix/store/7l68hmbqhmxib2wlmwyd5ib5czfyb25g-bzip2-1.0.6.0.1 /nix/store/9ikwdf54fsq002g4d36vp6r6qvwk5n3a-xz-5.2.5-bin /nix/store/f2l18n620bw7cmvrs1vh35kry3nrkfy0-xz-5.2.5 /nix/store/kl6lr3czkbnr6m5crcy8ffwfzbj8a22i-bash-4.4-p23 /nix/store/kl6lr3czkbnr6m5crcy8ffwfzbj8a22i-bash-4.4-p23 /nix/store/c1gipyfc161lm0fqfg65k4ak7yl2i17i-binutils-2.34 /nix/store/c1gipyfc161lm0fqfg65k4ak7yl2i17i-binutils-2.34 /nix/store/87ipxqn2ah5ba282cnip9z4mm3dbczjc-coreutils-8.32 /nix/store/87ipxqn2ah5ba282cnip9z4mm3dbczjc-coreutils-8.32 /nix/store/ck3kak4rp8j3649bwy0vk114qpy53q7g-diffutils-3.7 /nix/store/ck3kak4rp8j3649bwy0vk114qpy53q7g-diffutils-3.7 /nix/store/87l6ag9v5pa96axxg756hj4pfcajsmbh-findutils-4.7.0 /nix/store/87l6ag9v5pa96axxg756hj4pfcajsmbh-findutils-4.7.0 /nix/store/vnf5x5ggba946dpan66r1npqj4slrinw-gawk-5.1.0 /nix/store/vnf5x5ggba946dpan66r1npqj4slrinw-gawk-5.1.0 /nix/store/1pms77gm9l4rw8k180wmklagcwsya605-gnumake-4.3 /nix/store/1pms77gm9l4rw8k180wmklagcwsya605-gnumake-4.3 /nix/store/6qyl2yyqjrciypyb8728llsm2mqrhqsp-gnused-4.8 /nix/store/6qyl2yyqjrciypyb8728llsm2mqrhqsp-gnused-4.8 /nix/store/vkbdjkwqf7qxhxwd86cik7nfdppj04n2-gnutar-1.32 /nix/store/vkbdjkwqf7qxhxwd86cik7nfdppj04n2-gnutar-1.32 /nix/store/ipd8n0w9pwhfg1hsna9gv1z46y3j76d2-gnugrep-3.6 /nix/store/ipd8n0w9pwhfg1hsna9gv1z46y3j76d2-gnugrep-3.6 /nix/store/40sgch0d26mqjmrfvg6iq9vmkj2m5b7g-patch-2.7.6 /nix/store/40sgch0d26mqjmrfvg6iq9vmkj2m5b7g-patch-2.7.6 /nix/store/cfvww6slpwnvz19hf169ickmjda4l6jb-patchelf-0.12 /nix/store/cfvww6slpwnvz19hf169ickmjda4l6jb-patchelf-0.12 /nix/store/1s7h0ll18n2988ax3hhg2pjyni5jrf36-ed-1.16 /nix/store/1s7h0ll18n2988ax3hhg2pjyni5jrf36-ed-1.16 /nix/store/sylh15pml1cphys55sy0dr28cp8c42d0-attr-2.4.48 /nix/store/vy7phrnzkbjvsf9j5bkg2f3z8mf0fabv-acl-2.2.53 /nix/store/nnqw9pa10j11r3jjc0fw2rwdipram6gc-zlib-1.2.11 /nix/store/v3aw5ip513vsh6r36fs07niv0b3f7f8l-pcre-8.44 /nix/store/0cbzck6xac05rpg8wnz943vmikl1ws1d-libidn2-2.3.0 /nix/store/48j0w68fyd6lpwwfkjghphxvysgjl84x-libunistring-0.9.10 /nix/store/vbr3p4y2azrgwq2s7h2hbip8x06w50z5-glibc-2.32-24 /nix/store/x4v6h4gcw5vb2d0dm86mwk6kvxiyvxf8-glibc-2.32-24-dev /nix/store/y0mjg5mwdvq7m1dwhpz46wjck76bvba5-glibc-2.32-24-bin /nix/store/5njhh92vrhq6bq7vk56pqzp4mspqc3nr-linux-headers-5.10.4 /nix/store/fws5qmgl882n7zva366sw207b4rmgcyz-binutils-wrapper-2.34 /nix/store/d2fqghv083a8q7j8sjb2m5q6i840h691-gcc-wrapper-10.2.0 /nix/store/5wfh94xwg2hd7n82j8klgn4nmf07is24-gcc-10.2.0 /nix/store/xsqxamd86mh21dk0b775acaqgdijgf6p-gcc-10.2.0-lib /nix/store/xxwq0vv7pjinrr6gjfl2wv54qiaq1zsb-expand-response-params /nix/store/cfvww6slpwnvz19hf169ickmjda4l6jb-patchelf-0.12 /nix/store/mjjy30kxz775bhhi6j9phw81qh6dsbrf-move-docs.sh /nix/store/kxw6q8v6isaqjm702d71n2421cxamq68-make-symlinks-relative.sh /nix/store/cl3qd985p1yxyfkj96v0hqxiy3w69xq5-compress-man-pages.sh /nix/store/iw4rfvg0cmk6nyaprhmn4avrzmlwg9mf-strip.sh /nix/store/6k829xz9v508ncxfcf2p1b8g1wq54946-patch-shebangs.sh /nix/store/cickvswrvann041nqxb0rxilc46svw1n-prune-libtool-files.sh /nix/store/8zxndz5ag0p6s526c2xyllhk1nrn4c3i-audit-tmpdir.sh /nix/store/c8n9kcdddp9np665xz6ri61b383nxvz8-move-systemd-user-units.sh /nix/store/1i5y55x4b4m9qkx5dqbmr1r6bvrqbanw-multiple-outputs.sh /nix/store/kd4xwxjpjxi71jkm6ka0np72if9rm3y0-move-sbin.sh /nix/store/fyaryjvghbkpfnsyw97hb3lyb37s1pd6-move-lib64.sh /nix/store/ngg1cv31c8c7bcm2n8ww4g06nq7s4zhm-set-source-date-epoch-to-latest.sh /nix/store/pr6nzbscdpg94yvr151lrif2kg0csx7b-reproducible-builds.sh ←/nix/store/d2fqghv083a8q7j8sjb2m5q6i840h691-gcc-wrapper-10.2.0←
      ''
      defaultNativeBuildInputs=''
          /nix/store/cfvww6slpwnvz19hf169ickmjda4l6jb-patchelf-0.12 /nix/store/mjjy30kxz775bhhi6j9phw81qh6dsbrf-move-docs.sh /nix/store/kxw6q8v6isaqjm702d71n2421cxamq68-make-symlinks-relative.sh /nix/store/cl3qd985p1yxyfkj96v0hqxiy3w69xq5-compress-man-pages.sh /nix/store/iw4rfvg0cmk6nyaprhmn4avrzmlwg9mf-strip.sh /nix/store/6k829xz9v508ncxfcf2p1b8g1wq54946-patch-shebangs.sh /nix/store/cickvswrvann041nqxb0rxilc46svw1n-prune-libtool-files.sh /nix/store/8zxndz5ag0p6s526c2xyllhk1nrn4c3i-audit-tmpdir.sh /nix/store/c8n9kcdddp9np665xz6ri61b383nxvz8-move-systemd-user-units.sh /nix/store/1i5y55x4b4m9qkx5dqbmr1r6bvrqbanw-multiple-outputs.sh /nix/store/kd4xwxjpjxi71jkm6ka0np72if9rm3y0-move-sbin.sh /nix/store/fyaryjvghbkpfnsyw97hb3lyb37s1pd6-move-lib64.sh /nix/store/ngg1cv31c8c7bcm2n8ww4g06nq7s4zhm-set-source-date-epoch-to-latest.sh /nix/store/pr6nzbscdpg94yvr151lrif2kg0csx7b-reproducible-builds.sh ←/nix/store/d2fqghv083a8q7j8sjb2m5q6i840h691-gcc-wrapper-10.2.0←
      ''
• Skipping environment comparison

• I tried building something without flakes with haskellNix.allOverlays.combined-eval-on-build and haskellNix.overlays respectively and the outputs were identical. So that shouldn't be the problem.
• I also tried building some output that doesn't use haskell at all and it's the same for flakes/non-flakes builds.

@michaelpj @angerman do you have an ideas what this might be and what else i could perhaps try?