### Preamble¶

First, import what we need.

In [1]:

    from joy.joy import run
    from joy.library import initialize
    from joy.utils.stack import stack_to_string
    from joy.utils.pretty_print import TracePrinter

Define a dictionary, an initial stack, and two helper functions to run Joy code and print results for us.

In [2]:

    D = initialize()
    S = ()

    def J(text):
        print stack_to_string(run(text, S, D)[0])

    def V(text):
        tp = TracePrinter()
        run(text, S, D, tp.viewer)
        tp.print_()

### Run some simple programs¶

In [3]:

    J('23 18 +')

    41

In [4]:

    J('45 30 gcd')

    15

### With Viewer¶

A `viewer` records each step of the evaluation of a Joy program. The `TracePrinter` has a facility for printing out a trace of the evaluation, one line per step. Each step is aligned to the current interpreter position, signified by a period separating the stack on the left from the pending expression ("continuation") on the right. I find these traces beautiful, like a kind of art.

In [5]:

    V('23 18 +')

          . 23 18 +
       23 . 18 +
    23 18 . +
       41 .

In [6]:

    V('45 30 gcd')

                                      . 45 30 gcd
                                   45 . 30 gcd
                                45 30 . gcd
                                45 30 . 1 [tuck modulus dup 0 >] loop pop
                              45 30 1 . [tuck modulus dup 0 >] loop pop
       45 30 1 [tuck modulus dup 0 >] . loop pop
                                45 30 . tuck modulus dup 0 > [tuck modulus dup 0 >] loop pop
                             30 45 30 . modulus dup 0 > [tuck modulus dup 0 >] loop pop
                                30 15 . dup 0 > [tuck modulus dup 0 >] loop pop
                             30 15 15 . 0 > [tuck modulus dup 0 >] loop pop
                           30 15 15 0 . > [tuck modulus dup 0 >] loop pop
                           30 15 True . [tuck modulus dup 0 >] loop pop
    30 15 True [tuck modulus dup 0 >] . loop pop
                                30 15 . tuck modulus dup 0 > [tuck modulus dup 0 >] loop pop
                             15 30 15 . modulus dup 0 > [tuck modulus dup 0 >] loop pop
                                 15 0 . dup 0 > [tuck modulus dup 0 >] loop pop
                               15 0 0 . 0 > [tuck modulus dup 0 >] loop pop
                             15 0 0 0 . > [tuck modulus dup 0 >] loop pop
                           15 0 False . [tuck modulus dup 0 >] loop pop
    15 0 False [tuck modulus dup 0 >] . loop pop
                                 15 0 . pop
                                   15 .

Here's a longer trace.

In [7]:

    V('96 27 gcd')

                                      . 96 27 gcd
                                   96 . 27 gcd
                                96 27 . gcd
                                96 27 . 1 [tuck modulus dup 0 >] loop pop
                              96 27 1 . [tuck modulus dup 0 >] loop pop
       96 27 1 [tuck modulus dup 0 >] . loop pop
                                96 27 . tuck modulus dup 0 > [tuck modulus dup 0 >] loop pop
                             27 96 27 . modulus dup 0 > [tuck modulus dup 0 >] loop pop
                                27 15 . dup 0 > [tuck modulus dup 0 >] loop pop
                             27 15 15 . 0 > [tuck modulus dup 0 >] loop pop
                           27 15 15 0 . > [tuck modulus dup 0 >] loop pop
                           27 15 True . [tuck modulus dup 0 >] loop pop
    27 15 True [tuck modulus dup 0 >] . loop pop
                                27 15 . tuck modulus dup 0 > [tuck modulus dup 0 >] loop pop
                             15 27 15 . modulus dup 0 > [tuck modulus dup 0 >] loop pop
                                15 12 . dup 0 > [tuck modulus dup 0 >] loop pop
                             15 12 12 . 0 > [tuck modulus dup 0 >] loop pop
                           15 12 12 0 . > [tuck modulus dup 0 >] loop pop
                           15 12 True . [tuck modulus dup 0 >] loop pop
    15 12 True [tuck modulus dup 0 >] . loop pop
                                15 12 . tuck modulus dup 0 > [tuck modulus dup 0 >] loop pop
                             12 15 12 . modulus dup 0 > [tuck modulus dup 0 >] loop pop
                                 12 3 . dup 0 > [tuck modulus dup 0 >] loop pop
                               12 3 3 . 0 > [tuck modulus dup 0 >] loop pop
                             12 3 3 0 . > [tuck modulus dup 0 >] loop pop
                            12 3 True . [tuck modulus dup 0 >] loop pop
     12 3 True [tuck modulus dup 0 >] . loop pop
                                 12 3 . tuck modulus dup 0 > [tuck modulus dup 0 >] loop pop
                               3 12 3 . modulus dup 0 > [tuck modulus dup 0 >] loop pop
                                  3 0 . dup 0 > [tuck modulus dup 0 >] loop pop
                                3 0 0 . 0 > [tuck modulus dup 0 >] loop pop
                              3 0 0 0 . > [tuck modulus dup 0 >] loop pop
                            3 0 False . [tuck modulus dup 0 >] loop pop
     3 0 False [tuck modulus dup 0 >] . loop pop
                                  3 0 . pop
                                    3 .