Flake that works in repl, but not when building

2022-10-21 08:59:33

I posted this on the NixOS discourse yesterday, but thought I'd widen the net a bit.

The following flake.nix works fine when I load it using nix repl, but not when I try to build it with nix build.

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      with nixpkgs.legacyPackages.x86_64-linux;
      let
        isArthur = pkg:
          pkg.name == "arthur" && pkg.os == "linux" && pkg.arch == "amd64";
      in {
        packages.arthur = stdenv.mkDerivation rec {
          pname = "arthur";
          src = fetchurl {
            url =
              "s3://manifests/stable/manifest.json?region=eu-central-1";
            sha256 =
              "sha256:17bd9djdp5401ixglbw7jxw4a06w01imcc36n3dzrxiapfnarrny";
          };
          manifest = builtins.fromJSON (builtins.readFile src.outPath);
          version = manifest.version;
          arthur_pkg =
            builtins.elemAt (builtins.filter isArthur manifest.packages) 0;
          dontUnpack = true;
          dontBuild = true;

          installPhase = ''
            echo $version
          '';
        };

        defaultPackage = self.packages.${system}.arthur;
      });
}

In nix repl:

% nix repl
Welcome to Nix 2.6.0. Type :? for help.

nix-repl> :lf .
warning: Git tree '/home/haddock/projs/arthur-v2' is dirty
Added 10 variables.

nix-repl> outputs.defaultPackage.x86_64-linux.arthur_pkg
[0/1 built] { arch = "amd64"; executable = true; name = "arthur"; os = "linux"; public = false; sha256 = "10ffce33013f8106a51f0b06812df17876b1197aca97b78f1b4b248b072c6dd0"; size = 5283721; }

but when trying to build it:

% nix build
warning: Git tree '/home/haddock/projs/arthur-v2' is dirty
error: cannot coerce a set to a string

       at /nix/store/cxdpw8siygg8vp9w07i703lvkgm9889z-source/flake.nix:24:11:

           23|           version = manifest.version;
           24|           arthur_pkg =
             |           ^
           25|             builtins.elemAt (builtins.filter isArthur manifest.packages) 0;
(use '--show-trace' to show detailed location information)

I'm not sure what's going on here, why the attempt to coerce it to a string a all?

Sridhar Ratnakumar

2022-10-22 15:12:04

What's arthur_pkg?

2022-10-22 23:02:30

It's one on the JSON objects that can be found in the manifest.json file.

The details for parsing are in the first code block, and you can see the actual JSON object in the second code block.

David Costa

2022-11-14 14:33:29

Did you find the answer?

2022-11-19 20:52:24

Yes, I did. It's a quirk of mkDerivation it expects unknown keys to have string values.

Flake that works in repl, but not when building - Nix