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, as it gives the impression that its calling the list itself a set or a function.

It's especially confusing when you have an expression where you think you have excluded sets and you still end up with the error in the else branch:

$ nix eval --expr 'let f = (x: if builtins.isAttrs x then "{ ... }" else toString x); in f [ {} ]'
error: --- TypeError -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- nix
at: (1:55) from string

1| let f = (x: if builtins.isAttrs x then "{ ... }" else toString x); in f [ {} ]
|                                                       ^

cannot coerce a set to a string

Steps To Reproduce

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

nix-env --version output

nix-env (Nix) 2.4pre20200721_ff314f1

[stale]

I marked this as stale due to inactivity. → More info

[milahu]

i got error: cannot coerce a function to a string for passing a function to stdenv.mkDerivation

stdenv.mkDerivation {
  someFunction = x: x;
}

mkDerivation hidden in abstractions

# nix-build test.nix

with import <nixpkgs> {};
let
  npmlock2nix = rec {

    node_modules = # TRACE 7
      { src
      , lockfilePostprocess ? x: x
      , ...
      }
      @ args # TRACE 8
      :
      let
        lockfile = lockfilePostprocess {};
      in
      stdenv.mkDerivation ( # TRACE 9
        {
          version = lockfile.version;
          pname = lockfile.name;
        }
        //
        args # TRACE 10
      );

    build = # TRACE 3
      { src
      , node_modules_attrs ? {} # TRACE 4
      }:
      node_modules ( # TRACE 6
        { inherit src; }
        //
        node_modules_attrs # TRACE 5
      );
  };
in

npmlock2nix.build rec { # TRACE 2
  src = /tmp;
  node_modules_attrs = {

    # the function lockfilePostprocess will cause the error:
    # cannot coerce a function to a string
    # cos attributes for derivation must be strings
    # to reproduce the bug, lockfilePostprocess is NOT removed
    # via cleanArgs in npmlock2nix/internal.nix

    lockfilePostprocess = lockfileSet: ( # TRACE 1
      lockfileSet // {
        version = "1.2.3";
        name = "some-package";
      }
    );
  };
}

--show-trace is confusing

       at /tmp/test.nix:41:5:

           40|     # to reproduce the bug, lockfilePostprocess is NOT removed via cleanArgs in npmlock2nix/internal.nix
           41|     lockfilePostprocess = lockfileSet: (
             |     ^
           42|       lockfileSet // {

       … while evaluating the attribute 'lockfilePostprocess' of the derivation 'some-package-1.2.3'

       at /nix/store/hsyhx1ibbj3glm5b447hjp3ykbcdk6gl-nixos-21.11pre312335.8d8a28b47b7/nixos/pkgs/stdenv/generic/make-derivation.nix:205:7:

          204|     // (lib.optionalAttrs (attrs ? name || (attrs ? pname && attrs ? version)) {
          205|       name =
             |       ^
          206|         let

derivation should validate its attributes (must be strings)

or, as a proper fix: --show-trace should show more of the call stack:

• the call to toString
• the call to derivation
• the call to stdenv.mkDerivation

[stale]

I marked this as stale due to inactivity. → More info

[martin-braun]

I hate when this error appears, it's confusing and the message doesn't help to solve the issue. --show-trace will not go far enough back to see the origin of the issue.

The issue was that I did import ./whatever.nix instead of ./whatever.nix within the imports list.