What language is flake.nix written in?

[dasJ]

This issue may sound silly but it's actually not clear to me what language they are written in.
The syntax and file ending both look like Nix but the manual doesn't apply most of the time.
The manual makes it seem like this should work:

{
  description = "Testflake";
  outputs = import ./outputs.nix;
}

but running that results in

error: expected a function but got a thunk at /nix/store/lm4pyqnkbiwvp2kxjd64nqb2xkdxmbfx-source/flake.nix:4:3

So what language are flake.nix files written in (is there any specification or documentation) and why is that language not Nix?

[edolstra]

It's a subset of Nix that doesn't allow computation in the flake metadata attributes. So e.g. outputs cannot be a function application like import ./outputs.nix, it must be a function directly outputs = { bla }: .... This is to prevent arbitrarily complex, possibly non-terminating computations while querying flake metadata.

This should be documented in the flake manpage.

[stale]

I marked this as stale due to inactivity. → More info

[dasJ]

Will care about this when the feature gets remotely usable for my use case so please leave this open

[exarkun]

I ran into this and was quite surprised and confused for about half an hour. I used let ... in ... at the top of my flake.nix and the resulting error message, "error: file '/nix/store/91d71n4sxkxnby3c3pbc8is4xm7a70r9-source/flake.nix' must be an attribute set" had me scratching my head (builtins.typeOf said it evaluated to an attribute set) and looking all over for someplace I had put some non-set type into a set position.

I didn't believe the error message was worded accurately and precisely because I am used to Nix error messages that tell me one type was expected in place of another with little or no further location information and I thought this was one of those cases.

An error message something like "file '...' must be written using the limited flake expression language" might have helped me discover my mistake more quickly.

Also, none of the docs I read on my way to learning about flakes said anything about this limited expression language so apart from tripping over this error I'm not sure how I would have learned about this.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/can-i-use-flakes-within-a-git-repo-without-committing-flake-nix/18196/33