error:
         … while evaluating the error message passed to builtin.throw

         error: cannot coerce a list to a string: [ ]

We expect to be able to see locals like r in the debugger:

  nix-repl> r
  [ ]

  nix-repl> :env
  Env level 0
  static: x r

  Env level 1
  builtins true false null scopedImport import isNull break abort throw derivationStrict placeholder baseNameOf dirOf removeAttrs map toString fetchMercurial fetchTree fetchTarball fetchGit fromTOML derivation