# Type Checking

    import logging, sys

    logging.basicConfig(
      format='%(message)s',
      stream=sys.stdout,
      level=logging.INFO,
      )

    from joy.utils.polytypes import (
        doc_from_stack_effect,
        infer,
        reify,
        unify,
        FUNCTIONS,
        JoyTypeError,
    )

    D = FUNCTIONS.copy()
    del D['product']
    globals().update(D)

## An Example

    fi, fo = infer(pop, swap, rolldown, rrest, ccons)[0]

    25 (--) ∘ pop swap rolldown rrest ccons
    28 (a1 --) ∘ swap rolldown rrest ccons
    31 (a3 a2 a1 -- a2 a3) ∘ rolldown rrest ccons
    34 (a4 a3 a2 a1 -- a2 a3 a4) ∘ rrest ccons
    37 ([a4 a5 ...1] a3 a2 a1 -- a2 a3 [...1]) ∘ ccons
    40 ([a4 a5 ...1] a3 a2 a1 -- [a2 a3 ...1]) ∘

    print doc_from_stack_effect(fi, fo)

    ([a4 a5 ...1] a3 a2 a1 -- [a2 a3 ...1])

    from joy.parser import text_to_expression
    from joy.utils.stack import stack_to_string

    e = text_to_expression('0 1 2 [3 4]')  # reverse order
    print stack_to_string(e)

    [3 4] 2 1 0

    u = unify(e, fi)[0]
    u

    {a1: 0, a2: 1, a3: 2, a4: 3, a5: 4, s2: (), s1: ()}

    g = reify(u, (fi, fo))
    print doc_from_stack_effect(*g)

    (... [3 4 ] 2 1 0 -- ... [1 2 ])

## Unification Works “in Reverse”

    e = text_to_expression('[2 3]')

    u = unify(e, fo)[0]  # output side, not input side
    u

    {a2: 2, a3: 3, s2: (), s1: ()}

    g = reify(u, (fi, fo))
    print doc_from_stack_effect(*g)

    (... [a4 a5 ] 3 2 a1 -- ... [2 3 ])

## Failing a Check

    fi, fo = infer(dup, mul)[0]

    25 (--) ∘ dup mul
    28 (a1 -- a1 a1) ∘ mul
    31 (f1 -- f2) ∘
    31 (i1 -- i2) ∘

    e = text_to_expression('"two"')
    print stack_to_string(e)

    'two'

    try:
        unify(e, fi)
    except JoyTypeError, err:
        print err

    Cannot unify 'two' and f1.