nix eval --derivation ignores the --derivation flag

[lilyball]

Describe the bug

If I run nix eval with an installable that evaluates to a derivation, passing the --derivation flag does nothing. I expected it to evaluate to the derivation path (with or without the --raw flag). If I'm evaluating a single derivation I can add .drvPath myself (if I can remember to do so), but I always forget. And if I'm evaluating to some other structure that contains nested derivations that makes it much harder.

Steps To Reproduce

1. nix eval --raw --derivation nixpkgs#hello
2. nix eval --derivation nixpkgs#hello
3. nix eval --derivation nixpkgs#bat-extras (bat-extras is an attrset that contains derivations; also recurseForDerivations = true but that shouldn't matter here)
4. nix eval --derivation --expr 42

Expected behavior

(1) should print the same thing as nix eval --raw nixpkgs#hello.drvPath. Instead it prints the same thing as nix eval --raw nixpkgs#hello.

(2) should print some representation of the derivation. This could just be the drvPath (without quotes, this isn't a string), or perhaps do like nix repl and print e.g. «derivation /nix/store/bdxy7dq6l9xxpl2k2jx4sckgn93rhi0v-hello-2.10.drv».

(3) should print the attrset, except every nested derivation should be printed as a derivation instead of evaluated. I consider this a compelling reason to use syntax like «derivation /nix/store/bdxy7dq6l9xxpl2k2jx4sckgn93rhi0v-hello-2.10.drv» instead of bare paths, as that makes it clear what the type here is.

(4) is included because it involves no derivations. It should behave identically to nix eval --expr 42 as a result (as opposed to being an error). This just illustrates that the --derivation flag should serve to modify how derivations are evaluated rather than requiring the installable/expression to be a derivation or set of derivations.

I suppose another way to look at it is the --derivation flag should be roughly equivalent to running some sort of --apply 'x: mapRecursively (value: if isDerivation value then "«derivation ${value.drvPath}»" else value) (running after any actual --apply, and removing the quotes from the «derivation …» output, i.e. do this at the nix eval level instead of literally wrapping Nix code around it).

nix --version output

nix (Nix) 2.5.1

[pwaller]

I see this with nix 2.4, and it's confusing.

[andreabedini]

I believe this is a duplicate of #7261 (or at least releated).

As of version 2.15.1, nix does not accept eval --derivation anymore.

❯ nix eval --derivation nixpkgs#hello
error: unrecognised flag '--derivation'
Try 'nix --help' for more information.