.. code:: ipython2

    from notebook_preamble import D, DefinitionWrapper, J, V, define

Recursion Combinators
=====================

This article describes the ``genrec`` combinator, how to use it, and
several generic specializations.

::

                         [if] [then] [rec1] [rec2] genrec
   ---------------------------------------------------------------------
      [if] [then] [rec1 [[if] [then] [rec1] [rec2] genrec] rec2] ifte

From “Recursion Theory and Joy” (j05cmp.html) by Manfred von Thun:

   “The genrec combinator takes four program parameters in addition to
   whatever data parameters it needs. Fourth from the top is an if-part,
   followed by a then-part. If the if-part yields true, then the
   then-part is executed and the combinator terminates. The other two
   parameters are the rec1-part and the rec2-part. If the if-part yields
   false, the rec1-part is executed. Following that the four program
   parameters and the combinator are again pushed onto the stack bundled
   up in a quoted form. Then the rec2-part is executed, where it will
   find the bundled form. Typically it will then execute the bundled
   form, either with i or with app2, or some other combinator.”

Designing Recursive Functions
-----------------------------

The way to design one of these is to fix your base case and test and
then treat ``R1`` and ``R2`` as an else-part “sandwiching” a quotation
of the whole function.

For example, given a (general recursive) function ``F``:

::

   F == [I] [T] [R1]   [R2] genrec
     == [I] [T] [R1 [F] R2] ifte

If the ``[I]`` predicate is false you must derive ``R1`` and ``R2``
from:

::

   ... R1 [F] R2

Set the stack arguments in front and figure out what ``R1`` and ``R2``
have to do to apply the quoted ``[F]`` in the proper way.

Primitive Recursive Functions
-----------------------------

Primitive recursive functions are those where ``R2 == i``.

::

   P == [I] [T] [R] primrec
     == [I] [T] [R [P] i] ifte
     == [I] [T] [R P] ifte

`Hylomorphism <https://en.wikipedia.org/wiki/Hylomorphism_%28computer_science%29>`__
------------------------------------------------------------------------------------

A
`hylomorphism <https://en.wikipedia.org/wiki/Hylomorphism_%28computer_science%29>`__
is a recursive function ``H :: A -> C`` that converts a value of type
``A`` into a value of type ``C`` by means of:

-  A generator ``G :: A -> (B, A)``
-  A combiner ``F :: (B, C) -> C``
-  A predicate ``P :: A -> Bool`` to detect the base case
-  A base case value ``c :: C``
-  Recursive calls (zero or more); it has a “call stack in the form of a
   cons list”.

It may be helpful to see this function implemented in imperative Python
code.

.. code:: ipython2

    def hylomorphism(c, F, P, G):
        '''Return a hylomorphism function H.'''
    
        def H(a):
            if P(a):
                result = c
            else:
                b, aa = G(a)
                result = F(b, H(aa))  # b is stored in the stack frame during recursive call to H().
            return result
    
        return H

Cf. `“Bananas, Lenses, & Barbed
Wire” <http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.41.125>`__

Note that during evaluation of ``H()`` the intermediate ``b`` values are
stored in the Python call stack. This is what is meant by “call stack in
the form of a cons list”.

Hylomorphism in Joy
-------------------

We can define a combinator ``hylomorphism`` that will make a
hylomorphism combinator ``H`` from constituent parts.

::

   H == [P] c [G] [F] hylomorphism

The function ``H`` is recursive, so we start with ``ifte`` and set the
else-part to some function ``J`` that will contain a quoted copy of
``H``. (The then-part just discards the leftover ``a`` and replaces it
with the base case value ``c``.)

::

   H == [P] [pop c] [J] ifte

The else-part ``J`` gets just the argument ``a`` on the stack.

::

   a J
   a G              The first thing to do is use the generator G
   aa b             which produces b and a new aa
   aa b [H] dip     we recur with H on the new aa
   aa H b F         and run F on the result.

This gives us a definition for ``J``.

::

   J == G [H] dip F

Plug it in and convert to genrec.

::

   H == [P] [pop c] [G [H] dip F] ifte
   H == [P] [pop c] [G]   [dip F] genrec

This is the form of a hylomorphism in Joy, which nicely illustrates that
it is a simple specialization of the general recursion combinator.

::

   H == [P] c [G] [F] hylomorphism == [P] [pop c] [G] [dip F] genrec

Derivation of ``hylomorphism`` combinator
-----------------------------------------

Now we just need to derive a definition that builds the ``genrec``
arguments out of the pieces given to the ``hylomorphism`` combinator.

::

      [P]      c  [G]     [F] hylomorphism
   ------------------------------------------
      [P] [pop c] [G] [dip F] genrec

Working in reverse:

-  Use ``swoncat`` twice to decouple ``[c]`` and ``[F]``.
-  Use ``unit`` to dequote ``c``.
-  Use ``dipd`` to untangle ``[unit [pop] swoncat]`` from the givens.

So:

::

   H == [P] [pop c]              [G]                  [dip F] genrec
        [P] [c]    [pop] swoncat [G]        [F] [dip] swoncat genrec
        [P] c unit [pop] swoncat [G]        [F] [dip] swoncat genrec
        [P] c [G] [F] [unit [pop] swoncat] dipd [dip] swoncat genrec

At this point all of the arguments (givens) to the hylomorphism are to
the left so we have a definition for ``hylomorphism``:

::

   hylomorphism == [unit [pop] swoncat] dipd [dip] swoncat genrec

.. code:: ipython2

    define('hylomorphism == [unit [pop] swoncat] dipd [dip] swoncat genrec')

Example: Finding `Triangular Numbers <https://en.wikipedia.org/wiki/Triangular_number>`__
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let’s write a function that, given a positive integer, returns the sum
of all positive integers less than that one. (In this case the types
``A``, ``B`` and ``C`` are all ``int``.)

To sum a range of integers from 0 to *n* - 1:

-  ``[P]`` is ``[1 <=]``
-  ``c`` is ``0``
-  ``[G]`` is ``[-- dup]``
-  ``[F]`` is ``[+]``

.. code:: ipython2

    define('triangular_number == [1 <=] 0 [-- dup] [+] hylomorphism')

Let’s try it:

.. code:: ipython2

    J('5 triangular_number')


.. parsed-literal::

    10


.. code:: ipython2

    J('[0 1 2 3 4 5 6] [triangular_number] map')


.. parsed-literal::

    [0 0 1 3 6 10 15]


Four Specializations
--------------------

There are at least four kinds of recursive combinator, depending on two
choices. The first choice is whether the combiner function ``F`` should
be evaluated during the recursion or pushed into the pending expression
to be “collapsed” at the end. The second choice is whether the combiner
needs to operate on the current value of the datastructure or the
generator’s output, in other words, whether ``F`` or ``G`` should run
first in the recursive branch.

::

   H1 ==        [P] [pop c] [G             ] [dip F] genrec
   H2 == c swap [P] [pop]   [G [F]    dip  ] [i]     genrec
   H3 ==        [P] [pop c] [  [G] dupdip  ] [dip F] genrec
   H4 == c swap [P] [pop]   [  [F] dupdip G] [i]     genrec

The working of the generator function ``G`` differs slightly for each.
Consider the recursive branches:

::

   ... a G [H1] dip F                w/ a G == a′ b

   ... c a G [F] dip H2                 a G == b  a′

   ... a [G] dupdip [H3] dip F          a G == a′

   ... c a [F] dupdip G H4              a G == a′

The following four sections illustrate how these work, omitting the
predicate evaluation.

``H1``
~~~~~~

::

   H1 == [P] [pop c] [G] [dip F] genrec

Iterate n times.

::

   ... a  G [H1] dip F
   ... a′ b [H1] dip F
   ... a′ H1 b F
   ... a′ G [H1] dip F b F
   ... a″ b′ [H1] dip F b F
   ... a″ H1 b′ F b F
   ... a″ G [H1] dip F b′ F b F
   ... a‴ b″ [H1] dip F b′ F b F
   ... a‴ H1 b″ F b′ F b F
   ... a‴ pop c b″ F b′ F b F
   ... c b″ F b′ F b F
   ... d      b′ F b F
   ... d′          b F
   ... d″

This form builds up a pending expression (continuation) that contains
the intermediate results along with the pending combiner functions. When
the base case is reached the last term is replaced by the identity value
``c`` and the continuation “collapses” into the final result using the
combiner ``F``.

``H2``
~~~~~~

When you can start with the identity value ``c`` on the stack and the
combiner ``F`` can operate as you go using the intermediate results
immediately rather than queuing them up, use this form. An important
difference is that the generator function must return its results in the
reverse order.

::

   H2 == c swap [P] [pop] [G [F] dip] primrec

   ... c a G  [F] dip H2
   ... c b a′ [F] dip H2
   ... c b F a′ H2
   ... d     a′ H2
   ... d a′ G  [F] dip H2
   ... d b′ a″ [F] dip H2
   ... d b′ F a″ H2
   ... d′     a″ H2
   ... d′ a″ G  [F] dip H2
   ... d′ b″ a‴ [F] dip H2
   ... d′ b″ F a‴ H2
   ... d″      a‴ H2
   ... d″ a‴ pop
   ... d″

``H3``
~~~~~~

If you examine the traces above you’ll see that the combiner ``F`` only
gets to operate on the results of ``G``, it never “sees” the first value
``a``. If the combiner and the generator both need to work on the
current value then ``dup`` must be used, and the generator must produce
one item instead of two (the b is instead the duplicate of a.)

::

   H3 == [P] [pop c] [[G] dupdip] [dip F] genrec

   ... a [G] dupdip [H3] dip F
   ... a  G  a      [H3] dip F
   ... a′    a      [H3] dip F
   ... a′ H3 a               F
   ... a′ [G] dupdip [H3] dip F a F
   ... a′  G  a′     [H3] dip F a F
   ... a″     a′     [H3] dip F a F
   ... a″ H3  a′              F a F
   ... a″ [G] dupdip [H3] dip F a′ F a F
   ... a″  G    a″   [H3] dip F a′ F a F
   ... a‴       a″   [H3] dip F a′ F a F
   ... a‴ H3    a″            F a′ F a F
   ... a‴ pop c a″ F a′ F a F
   ...        c a″ F a′ F a F
   ...        d      a′ F a F
   ...        d′          a F
   ...        d″

``H4``
~~~~~~

And, last but not least, if you can combine as you go, starting with
``c``, and the combiner ``F`` needs to work on the current item, this is
the form:

::

   H4 == c swap [P] [pop] [[F] dupdip G] primrec

   ... c  a  [F] dupdip G H4
   ... c  a   F  a      G H4
   ... d         a      G H4
   ... d  a′              H4
   ... d  a′ [F] dupdip G H4
   ... d  a′  F  a′     G H4
   ... d′        a′     G H4
   ... d′ a″              H4
   ... d′ a″ [F] dupdip G H4
   ... d′ a″  F  a″     G H4
   ... d″        a″     G H4
   ... d″ a‴              H4
   ... d″ a‴ pop
   ... d″

Anamorphism
-----------

An anamorphism can be defined as a hylomorphism that uses ``[]`` for
``c`` and ``swons`` for ``F``. An anamorphic function builds a list of
values.

::

   A == [P] [] [G] [swons] hylomorphism

``range`` et. al. An example of an anamorphism is the ``range`` function which generates the list of integers from 0 to *n* - 1 given *n*.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Each of the above variations can be used to make four slightly different
``range`` functions.

``range`` with ``H1``
^^^^^^^^^^^^^^^^^^^^^

::

   H1 == [P]    [pop c]  [G]      [dip F]     genrec
      == [0 <=] [pop []] [-- dup] [dip swons] genrec

.. code:: ipython2

    define('range == [0 <=] [] [-- dup] [swons] hylomorphism')

.. code:: ipython2

    J('5 range')


.. parsed-literal::

    [4 3 2 1 0]


``range`` with ``H2``
^^^^^^^^^^^^^^^^^^^^^

::

   H2 == c  swap [P]    [pop] [G      [F]     dip] primrec
      == [] swap [0 <=] [pop] [-- dup [swons] dip] primrec

.. code:: ipython2

    define('range_reverse == [] swap [0 <=] [pop] [-- dup [swons] dip] primrec')

.. code:: ipython2

    J('5 range_reverse')


.. parsed-literal::

    [0 1 2 3 4]


``range`` with ``H3``
^^^^^^^^^^^^^^^^^^^^^

::

   H3 == [P]    [pop c]  [[G]  dupdip] [dip F]     genrec
      == [0 <=] [pop []] [[--] dupdip] [dip swons] genrec

.. code:: ipython2

    define('ranger == [0 <=] [pop []] [[--] dupdip] [dip swons] genrec')

.. code:: ipython2

    J('5 ranger')


.. parsed-literal::

    [5 4 3 2 1]


``range`` with ``H4``
^^^^^^^^^^^^^^^^^^^^^

::

   H4 == c  swap [P]    [pop] [[F]     dupdip G ] primrec
      == [] swap [0 <=] [pop] [[swons] dupdip --] primrec

.. code:: ipython2

    define('ranger_reverse == [] swap [0 <=] [pop] [[swons] dupdip --] primrec')

.. code:: ipython2

    J('5 ranger_reverse')


.. parsed-literal::

    [1 2 3 4 5]


Hopefully this illustrates the workings of the variations. For more
insight you can run the cells using the ``V()`` function instead of the
``J()`` function to get a trace of the Joy evaluation.

Catamorphism
------------

A catamorphism can be defined as a hylomorphism that uses
``[uncons swap]`` for ``[G]`` and ``[[] =]`` (or just ``[not]``) for the
predicate ``[P]``. A catamorphic function tears down a list term-by-term
and makes some new value.

::

   C == [not] c [uncons swap] [F] hylomorphism

.. code:: ipython2

    define('swuncons == uncons swap')  # Awkward name.

An example of a catamorphism is the sum function.

::

   sum == [not] 0 [swuncons] [+] hylomorphism

.. code:: ipython2

    define('sum == [not] 0 [swuncons] [+] hylomorphism')

.. code:: ipython2

    J('[5 4 3 2 1] sum')


.. parsed-literal::

    15


The ``step`` combinator
~~~~~~~~~~~~~~~~~~~~~~~

The ``step`` combinator will usually be better to use than
``catamorphism``.

.. code:: ipython2

    J('[step] help')


.. parsed-literal::

    Run a quoted program on each item in a sequence.
    ::
    
            ... [] [Q] . step
         -----------------------
                   ... .
    
    
           ... [a] [Q] . step
        ------------------------
                 ... a . Q
    
    
         ... [a b c] [Q] . step
      ----------------------------------------
                   ... a . Q [b c] [Q] step
    
    The step combinator executes the quotation on each member of the list
    on top of the stack.
    


.. code:: ipython2

    define('sum == 0 swap [+] step')

.. code:: ipython2

    J('[5 4 3 2 1] sum')


.. parsed-literal::

    15


Example: Factorial Function
---------------------------

For the Factorial function:

::

   H4 == c swap [P] [pop] [[F] dupdip G] primrec

With:

::

   c == 1
   F == *
   G == --
   P == 1 <=

.. code:: ipython2

    define('factorial == 1 swap [1 <=] [pop] [[*] dupdip --] primrec')

.. code:: ipython2

    J('5 factorial')


.. parsed-literal::

    120


Example: ``tails``
------------------

An example of a paramorphism for lists given in the `“Bananas…”
paper <http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.41.125>`__
is ``tails`` which returns the list of “tails” of a list.

::

       [1 2 3] tails
   --------------------
      [[] [3] [2 3]]

We can build as we go, and we want ``F`` to run after ``G``, so we use
pattern ``H2``:

::

   H2 == c swap [P] [pop] [G [F] dip] primrec

We would use:

::

   c == []
   F == swons
   G == rest dup
   P == not

.. code:: ipython2

    define('tails == [] swap [not] [pop] [rest dup [swons] dip] primrec')

.. code:: ipython2

    J('[1 2 3] tails')


.. parsed-literal::

    [[] [3] [2 3]]


Conclusion: Patterns of Recursion
---------------------------------

Our story so far…

Hylo-, Ana-, Cata-
~~~~~~~~~~~~~~~~~~

::

   H == [P  ] [pop c ] [G          ] [dip F        ] genrec
   A == [P  ] [pop []] [G          ] [dip swap cons] genrec
   C == [not] [pop c ] [uncons swap] [dip F        ] genrec

Para-, ?-, ?-
~~~~~~~~~~~~~

::

   P == c  swap [P  ] [pop] [[F        ] dupdip G          ] primrec
   ? == [] swap [P  ] [pop] [[swap cons] dupdip G          ] primrec
   ? == c  swap [not] [pop] [[F        ] dupdip uncons swap] primrec

Appendix: Fun with Symbols
--------------------------

::

   |[ (c, F), (G, P) ]| == (|c, F|) • [(G, P)]

`“Bananas, Lenses, & Barbed
Wire” <http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.41.125>`__

::

   (|...|)  [(...)]  [<...>]

I think they are having slightly too much fun with the symbols. However,
“Too much is always better than not enough.”
