Nixpkgs: `nix-shell -p agda` fails

[karljs]

Issue description

Opening a nix-shell with the agda package fails.

Steps to reproduce

nix-shell -p agda

Technical details

The error message is:

error: while evaluating the attribute 'buildInputs' of the derivation 'shell' at /nix/store/w8cnxvdyd3ngwn71pb1vdpiz3vj2xhv0-nixpkgs-19.03pre169106.1a88aa9e0cd/nixpkgs/pkgs/build-support/trivial-builders.nix:7:14:
cannot coerce a set to a string, at /nix/store/w8cnxvdyd3ngwn71pb1vdpiz3vj2xhv0-nixpkgs-19.03pre169106.1a88aa9e0cd/nixpkgs/pkgs/build-support/trivial-builders.nix:7:14

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

 - system: `"x86_64-linux"`
 - host os: `Linux 4.20.7-200.fc29.x86_64, Fedora, 29 (Workstation Edition)`
 - multi-user?: `no`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.2.1`
 - channels(karl): `"nixpkgs-19.03pre169106.1a88aa9e0cd"`
 - nixpkgs: `/home/karl/.nix-defexpr/channels/nixpkgs`

[davidak]

It also fails on NixOS:

[davidak@ethmoid:~]$ nix-shell -p agda
error: cannot coerce a set to a string, at /nix/store/yqhfqlplypysidkvqq3vfi5jf2qxd33l-nixos-18.09.2203.9bd45dddf81/nixos/pkgs/stdenv/generic/make-derivation.nix:177:11

---

• system: "x86_64-linux"
• host os: Linux 4.14.101, NixOS, 18.09.2203.9bd45dddf81 (Jellyfish)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.1.3
• channels(root): "nixos-18.09.2203.9bd45dddf81, nixos-hardware, nixos-unstable-19.03pre169108.36f31600749"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

[arianvp]

That's because agda is not a package but a set containing functions like mkDerivation to create agda packages. Agda itself is under haskellPackages.Agda

I don't think the agda stuff is documented anywhere. I found it confusing too.

[davidak]

That's because agda is not a package but a set containing functions

Maybe the error should say that?

[arianvp]

The error says that. "Cannot coerce set to a string" suggests that it's not a package but a set.

Same thing occurs, I think , if you try:

nix-shell -p haskellPackages

Im not sure if it's possible to customise error messages like that. Maybe someone else can chime in on that.

But the best way (apart from a better error message) to fix this issue is to add a section in the manual about how agda language support works; as it's currently undocumented

[arianvp]

Related: https://github.com/NixOS/nixpkgs/pull/39318

It would make more sense I think to expose agda under the agda attribute, and move the current agda attribute to something like agdaPackages. That would maybe also clear confusion.

[karljs]

Thanks for explaining this. I like the idea of changing to agdaPackages to make this more self-documenting.

[turion]

This has recently been fixed by #76653: nix-shell -I nixpkgs=channel:nixpkgs-unstable -p agda works.