shell.nix doesn't set proper library path

[zeratax]

It seems to me like the provided shell.nix doesn't set the library path correctly?

when I run

$ nix-shell
$ echo 'puts "Hello World!"' > hello_world.cr
$ crystal run hello_world.cr

I get

Showing last frame. Use --error-trace for full trace.

In hello_world.cr:1:1

 1 | puts "Hello World!"
     ^
Error: can't find file 'prelude'

If you're trying to require a shard:
- Did you remember to run `shards install`?
- Did you make sure you're running the compiler in the same directory as your shard.yml?

Which afaict is due to the library path not being set correctly, which seems to be confirmed when I run:

$ crystal env
CRYSTAL_CACHE_DIR=/home/kaine/.cache/crystal
CRYSTAL_PATH=lib
CRYSTAL_VERSION=1.0.0
CRYSTAL_LIBRARY_PATH=''
CRYSTAL_OPTS=''

my system:

• system: "x86_64-linux"
• host os: Linux 5.4.108, NixOS, 20.09.3732.91b77fe6942 (Nightingale)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.10
• channels(root): "nixos-20.09.3732.91b77fe6942, nixos-unstable-21.05pre279456.04a2b269d89"
• channels(kaine): "home-manager-20.09"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos