nix-repl> 1 + 1
  2

  nix-repl> :doc builtins.add
  Synopsis: builtins.add e1 e2

      Return the sum of the numbers e1 and e2.


  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