Attempting to coerce a list to a string may fail with confusing error "cannot coerce a set/function to a string"

[lboklin]

Describe the bug

Attempting to call toString on a list containing a set will produce the very confusing error:

cannot coerce a set to a string

or a list containing a function:

cannot coerce a function to a string

This can take a long time to figure out the first time it happens, given that the error focuses on the list, rather than the element in question.

To Reproduce
Steps to reproduce the behavior:

1. nix eval --expr 'toString [ (x: x) ]' or nix eval --expr 'toString [ {} ]'

Expected behavior
A more helpful error, such as

cannot coerce a list to a string because of one of its elements: cannot coerce a set to a string

Metadata

• system: "x86_64-linux"
• host os: Linux 5.7.16, NixOS, 20.09.20200820.c59ea8b (Nightingale)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.4pre20200721_ff314f1
• channels(root): "nixos-20.03.2806.cb1996818ed"
• channels(ludvig): "home-manager, nixpkgs-20.09pre238994.078b9f8aa68, nixos-19.09-19.09.2284.bf7c0f0461e"
• nixpkgs: /home/ludvig/.nix-defexpr/channels/nixpkgs

[jonringer]

this should probably be made in https://github.com/NixOS/nix as it relates to the nix cli, and not the nixpkgs package set

[lboklin]

this should probably be made in https://github.com/NixOS/nix as it relates to the nix cli, and not the nixpkgs package set

Good point.

[lboklin]

Opened at NixOS/nix#4000 instead.