[coderaptor]

But it seems to be doing it imperatively. I’d expect something like ‘nginxConf = pkgs.file “nginx.conf”, “contents”’ instead of ‘nginxConf = pkgs.writeText “nginx.conf”, “contents”’.

Not saying the system doesn’t apply this declaratively, but I find it difficult to intuit the above is checking for a state and applying changes only if necessary.

[reuben364]

One distinction in Nix vs Docker is that Nix has a dag structure as opposed to a singlely linked list structure of layers.

The "writeText" function produces a derivation (basically an atomic build recipe) that produces that file. The crux of nix is that you make deterministic derivations, and then you can always refer to the results of a derivation from the hash of the derivation and its inputs.

What nix adds is glue logic to chain these derivations together in a way that preserves reproducibility of the individual imperative, but deterministic, components.

Unless you are using something like recursive-nix, you can completely evaluate the nix expression without building any of the derivations.

[mikepurvis]

Also relevant to note that although Nix builds individual derivations imperatively (call this compiler, write this file, rename this directory), it completely controls all the inputs to that imperative process.

This is fundamentally different from a Dockerfile or Ansible script which have no idea what the "starting point" of the target environment is and are pretty much just mindlessly imposing mutations on top of whatever happens to already be there.