nix-repl> 1 + 1
  2

  nix-repl> :doc builtins.head
  Synopsis: builtins.head list

      Return the first element of a list; abort evaluation if
      the argument isn’t a list or is an empty list. You can
      test whether a list is empty by comparing it with [].

  nix-repl> f = a: "" + a

Expect the trace to not contain any traceback:

  nix-repl> f 2
  error:
         … while evaluating a path segment
           at «string»:1:10:
              1|  a: "" + a
               |          ^

         error: cannot coerce an integer to a string: 2

  nix-repl> :te
  showing error traces

Expect the trace to have traceback:

  nix-repl> f 2
  error:
         … from call site
           at «string»:1:1:
              1| f 2
               | ^

         … while calling anonymous lambda
           at «string»:1:2:
              1|  a: "" + a
               |  ^

         … while evaluating a path segment
           at «string»:1:10:
              1|  a: "" + a
               |          ^

         error: cannot coerce an integer to a string: 2

Turning it off should also work:

  nix-repl> :te
  not showing error traces

  nix-repl> f 2
  error:
         … while evaluating a path segment
           at «string»:1:10:
              1|  a: "" + a
               |          ^

         error: cannot coerce an integer to a string: 2