Print the value in error: cannot coerce messages

[9999years]

Motivation

This extends the error: cannot coerce a <TYPE> to a string message to print the value that could not be coerced. This helps with debugging by making it easier to track down where the value is being produced from, especially in errors with deep or unhelpful stack traces.

Context

Take two of #9553 now that #9606 is merged.
See #561.

Here's an example of a mistake I made that would have been much easier to fix with this patch. I encountered this error while converting a NixOS configuration that relied on builtins.currentSystem to work in a flake. There were several overlays which pinned packages to specific versions, like this:

let
  nixos_master = import (
    builtins.fetchGit {
      url = "https://github.com/NixOS/nixpkgs.git";
      ref = "master";
      rev = "0c0d57d79de06b36286b8c7551dc173e372b86ad";
    }
  ) {};
in
self: super: {
  opentelemetry-collector-contrib = nixos_master.opentelemetry-collector-contrib; # version 0.78.0
}

My first try added an inherit (config.nixpkgs) localSystem; to the import arguments, but that gave me an unhelpful error:

$ nix repl nixpkgs
nix-repl> nixpkgs = legacyPackages.x86_64-linux.path
nix-repl> system = lib.systems.elaborate "x86_64-linux"
nix-repl> import nixpkgs { inherit system; }
error:
       … while evaluating a branch condition

         at /nix/store/7g8wbm1z6948x0qma4hzkkb9a3xys76w-source/pkgs/stdenv/booter.nix:64:9:

           63|       go = pred: n:
           64|         if n == len
             |         ^
           65|         then rnul pred

       … while calling the 'length' builtin

         at /nix/store/7g8wbm1z6948x0qma4hzkkb9a3xys76w-source/pkgs/stdenv/booter.nix:62:13:

           61|     let
           62|       len = builtins.length list;
             |             ^
           63|       go = pred: n:

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: cannot coerce a set to a string

Note that the stack trace and error message don't make it clear which set couldn't be coerced to a string, or where the coercion failed. It's fairly easy to figure out what's gone wrong here, but in a repository with 14kloc it was significantly more difficult.

With this patch, the error message makes it clear what's happened (a system attrset has been used in place of a system string):

error: cannot coerce a set to a string: { aesSupport = «thunk»; avx2Support = «thunk»; avx512Support = «thunk»; avxSupport = «thunk»; canExecute = «thunk»; config = «thunk»; darwinArch = «thunk»; darwinMinVersion = «thunk»; darwinMinVersionVariable = «thunk»; darwinPlatform = «thunk»; «94 attributes elided»}

Priorities

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[roberth]

It seems that some comments from this review apply here as well.

[Ma27]

@9999years do you also have patches for the error message "attempt to call something which is not a function" lying around? Otherwise I'd be happy to write something for that as well :)

[9999years]

@Ma27

do you also have patches for the error message "attempt to call something which is not a function"

No but that sounds great. Go for it!

[roberth]

Thanks again @9999years for another nice improvement.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-01-22-nix-team-meeting-minutes-117/38838/1