[Nix-dev] Try to build first time: fails. Try to build again: succeeds. What the hell?

[Mateusz Kowalczyk]

Hello,

I've been trying to get a newer version of GHC under the GHC HEAD
package in nixpkgs.

I located some sources and packaged them up and sent a PR at [1].

As pointed out, those sources fail to compile. You can see the error
message right on the PR and I can replicate it. However, if I try to run
the build again, the error goes away and it all compiles.

Does anyone have any idea what the hell is going on? The way I'm trying
it is by having the updated package in nixpkgs, depending on
haskellPackages_ghcHead in default.nix for a project and then running
nix-shell.

I'm in dire need of having newer ‘HEAD’ in my package repository but
understandably no one will take it into nixpkgs if it fails to build
(the first time). I'm not the package maintainer to begin with so I have
very little idea what I'm doing to start with but the actual maintainers
haven't spoken up on the PR yet.

[1]: https://github.com/NixOS/nixpkgs/issues/2428
-- 
Mateusz K.