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

Welcome to the Functional Programming Zulip Chat Archive. You can join the chat here.

Magnus Therning

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;
        isArthur = pkg:
          pkg.name == "arthur" && pkg.os == "linux" && pkg.arch == "amd64";
      in {
        packages.arthur = stdenv.mkDerivation rec {
          pname = "arthur";
          src = fetchurl {
            url =
            sha256 =
          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?

Magnus Therning

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

Did you find the answer?

Magnus Therning

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