A function which returns a flake?

#1

Hello all,

I work at a company where we have multiple clients, and where the latest version of a product is obtainable by a bookmark (like a git branch, a pointer to a particular rev) in mercurial. So we have flakes that start with

{
  inputs = {
    our-product = hg+https://our-repo.com/our-product?ref=client-a;
  }
  ...
}

My question would be, is it possible to write a function which abstracts over a flake, so that the client bookmark - client-a above - can be passed as an argument to said function, and it then returns a flake? I’m assuming not, since the nix package manager presumably wants a fully-formed flake as an entrypoint, but I thought I’d ask.

1 Like
#2

No, there’s nothing like that. It would be pretty hard to make that interact sanely with the locking system.

#3

Short answer: no

Long answer:
There are a couple possibilities depending on what you want to do

  • You can use the existing flake with --override-input whenever you want to use something other then the predefined our-product
  • You can create another flake that overrides the current flake using inputs.*.follows. This allows you to define multiple instances of the existing flake and use it almost like a function
  • If the our-product is not a flake, you can make the current flake generic over its inputs and just apply it with different sources called with fetchhg, it is also possible if our-product is a flake with builtins.getFlake but you are probably better off with the previous solution if that’s the case.

On a side note, unquoted URLs are being deprecated as per RFC 45

2 Likes
#4

Thank you! That second approach seems like the most viable one.

#5

Could I check - the second option here, how would that work? If I understand correctly, you can only have one flake per thing you want to build, since:

  • You cannot have multiple flakes sitting side-by-side in the root of a repo, and
  • If you put flakes in a sub-directory of that repo, they cannot build code at a lower level of the repo - ie, you cannot do, say src = ../..
#6

If you put flakes in a sub-directory of that repo, they cannot build code at a lower level of the repo

You can get around that by having the flake in the sub-directory generic over the source dir, e.g. as a function argument

Another way is just to have two separate repositories for two separate flakes

#7

Now that I think about it, if you already need a function, you can just have one flake and apply that function to multiple inputs

#8

Sorry, I don’t quite get what you mean by that one. One flake would act as a single function, and then I would (somehow) pass it the inputs?

#9

What I meant is that you can access ../. by having the output of the flake in the subdirectory (I will call it a) accept a path, e.g., packages.x86_64-linux = src: stdenv.mkDerivation { inherit src; /* ... */ };, and the actual flake at the root level would use a.packages.x86_64-linux self since it has access to this directory. this example is overly simplified, you might actually want to do overrides instead so stuff like nix flake show would work

#10

Thank you - seems like there are some good options for solving this - I’ve actually gone with the approach of having a script to edit the lockfile, but appreciate your help.

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