nix-shell --pure usage doesn't work for the full niv API?

[rpearce]

Hi, I am so grateful for this tool and am soon to write some blog posts that detail using it for various types of projects!

Some notes before my question:

• ✅ niv works great as a system-wide install
• ✅ niv works great with nix-shell --packages niv --run "niv ..."

But when I use nix-shell --pure (a pure environment), I run into this issue:

λ nix-shell --pure --packages niv --run "niv add NixOS/nixpkgs-channels --name nixpkgs --branch nixpkgs-unstable"
Adding package nixpkgs
  FATAL: One or more packages failed to update:
  nixpkgs: nix-prefetch-url: readCreateProcessWithExitCode: runInteractiveProcess: exec: does not exist (No such file or directory)
ERROR: ExitFailure 1

The same thing happens if I go into interactive shell mode (naturally):

λ nix-shell --pure --packages niv

[nix-shell:/path/to/project]$ niv add NixOS/nixpkgs-channels --name nixpkgs --branch nixpkgs-unstable
Adding package nixpkgs
  FATAL: One or more packages failed to update:
  nixpkgs: nix-prefetch-url: readCreateProcessWithExitCode: runInteractiveProcess: exec: does not exist (No such file or directory)
ERROR: ExitFailure 1

However, if I use the drop command, that works fine (given I've added a package in a different way):

λ nix-shell --pure --packages niv --run "niv drop nixpkgs"
Dropping package: nixpkgs

Can you please help me figure out what I'm doing wrong here, or if this is an issue?

---

Here's what my example release.nix looks like for a haskell project:

{ compiler ? "ghc881" }:

let
  sources = import ./nix/sources.nix;
  pkgs = import sources.nixpkgs { config = { }; };
  haskellPackages = pkgs.haskell.packages.${compiler};
  project = haskellPackages.callPackage ./project.nix { };
in
  {
    project = project;

    shell = haskellPackages.shellFor {
      packages = p: with p; [
        project
      ];
      buildInputs = with haskellPackages; [
        cabal-install
        hlint
        ghcid
        ormolu
        niv
      ];
      withHoogle = true;
    };
  }

If I add pkgs.nix to buildInputs,

buildInputs = with haskellPackages; [
  cabal-install
  hlint
  ghcid
  ormolu
  niv
+ pkgs.nix
];

I get this response when running nix-shell --pure

[nix-shell:~/projects/timetrack-cli]$ niv add nixpkgs -o NixOS -r nixpkgs-channels -b nixpkgs-unstable
Adding package nixpkgs
  FATAL: Could not read the output of 'nix-prefetch-url'. This is a bug. Please create a
  ticket:
    https://github.com/nmattia/niv/issues/new
  Thanks! I'll buy you a beer.
  stdout:
  stderr:
  error: unable to download 'https://github.com/NixOS/nixpkgs-channels/archive/05f0934825c2a0750d4888c4735f9420c906b388.tar.gz': Problem with the SSL CA cert (path? access rights?) (77)
  FATAL: One or more packages failed to update:
  nixpkgs: ExitFailure 1
ERROR: ExitFailure 1

@utdemir mentioned that something like cacert might need to be something that is a runtime dependency for niv, and adding pkgs.nix and pkgs.cacert to the buildInputs got it working.

buildInputs = with haskellPackages; [
  cabal-install
  hlint
  ghcid
  ormolu
  niv
+ pkgs.nix
+ pkgs.cacert
];

---

In short, do pkgs.cacert and something else need to be runtime dependencies?

[jeff-hykin]

I had the same issue, and .nix and .cacert worked for me as well.

[nmattia]

Wops, missed the first notification! Yes, niv uses nix-prefetch-url, which is part of the pkgs.nix derivation. We could reimplement it but that'd open a can of worms.

I guess niv should depend on cacert and nix at runtime, but I'm not 100% sure what's the correct way to override a Haskell package in nixpkgs is; I'll ask around.

[rpearce]

@nmattia What are you trying to override, specifically? I know a couple of things about overriding haskell packages in nixpkgs

Example:

let
  sources = import ./nix/sources.nix;
in
{ compiler ? "ghc883"
, pkgs ? import sources.nixpkgs { }
}:

let
  inherit (pkgs.lib.trivial) flip pipe;
  inherit (pkgs.haskell.lib) appendPatch appendConfigureFlags;

  haskellPackages = pkgs.haskell.packages.${compiler}.override {
    overrides = hpNew: hpOld: {
      hakyll =
        pipe
           hpOld.hakyll
           [ (flip appendPatch ./hakyll.patch)
             (flip appendConfigureFlags [ "-f" "watchServer" "-f" "previewServer" ])
           ];

      # ...
    };
  };
in # ...

Also, lots of good stuff here for Haskell & Nix: https://github.com/Gabriel439/haskell-nix