GDB can almost be a dynamic computing environment:

  $ gdb ./txr
  GNU gdb [ ... ]
  [ ... ]
  Reading symbols from /home/kaz/txr/txr...done.
  (gdb) b eval
  Breakpoint 1 at 0x808f6b0: file eval.c, line 969.
  (gdb) r -p '(+ 2 2)'
  Starting program: /home/kaz/txr/txr -p '(+ 2 2)'

  Breakpoint 1, eval (form=0xb7fb919c, env=0xb7fb92ac, ctx_form=0xb7fb919c)
      at eval.c:969
  969     {
  (gdb) p d(form)
  (+ 2 2)
  $1 = void
  (gdb) p d(cons(form, form))
  ((+ 2 2) + 2 2)
  $2 = void
  (gdb) p d(cdr(form))
  (2 2)
  $3 = void
  (gdb) p d(cons(0x1, 0x1))
  (0 . 0)
  $4 = void
  (gdb) p cons(0x1, 0x0)
  $5 = (obj_t *) 0xb7fb921c
  (gdb) p cons(0x9, $5)
  $6 = (obj_t *) 0xb7fb920c
  (gdb) p cons(0x19, $6)
  $7 = (obj_t *) 0xb7fb91fc
  (gdb) p d($7)
  (6 2 0)

You can test functions that are not even called anywhere from the C code. The d function is such a function; it's only there for use out of GDB.

  (gdb) p (0)
  $10 = 0
  (gdb) p d(0)
  nil
  $11 = void
  (gdb) p t
  $12 = (val) 0xb7fe5eac
  (gdb) p d(t)
  t
  $13 = void
  (gdb) p typeof(t)
  $14 = (obj_t *) 0xb7fe5d3c
  (gdb) p d(typeof(t))
  sym
  $15 = void
  (gdb) p d(symbol_name(t))
  "t"
  $16 = void
  (gdb) p d(symbol_name(0))
  "nil"
  $17 = void

Take out the trash, and show it:

  (gdb) p gc()
  $18 = void
  (gdb) p d($7)
  #<garbage: 0xb7fb91fc>