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


A hylomorphism 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.

def hylomorphism(c, F, P, G):
    '''Return a hylomorphism function H.'''

    def H(a):
        if P(a):
            result = c
            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”

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.


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

Example: Finding Triangular Numbers

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 [+]

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

Let’s try it:

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


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″


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″


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″


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
define('range == [0 <=] [] [-- dup] [swons] hylomorphism')
J('5 range')
[4 3 2 1 0]

range with H2

H2 == c  swap [P]    [pop] [G      [F]     dip] primrec
   == [] swap [0 <=] [pop] [-- dup [swons] dip] primrec
define('range_reverse == [] swap [0 <=] [pop] [-- dup [swons] dip] primrec')
J('5 range_reverse')
[0 1 2 3 4]

range with H3

H3 == [P]    [pop c]  [[G]  dupdip] [dip F]     genrec
   == [0 <=] [pop []] [[--] dupdip] [dip swons] genrec
define('ranger == [0 <=] [pop []] [[--] dupdip] [dip swons] genrec')
J('5 ranger')
[5 4 3 2 1]

range with H4

H4 == c  swap [P]    [pop] [[F]     dupdip G ] primrec
   == [] swap [0 <=] [pop] [[swons] dupdip --] primrec
define('ranger_reverse == [] swap [0 <=] [pop] [[swons] dupdip --] primrec')
J('5 ranger_reverse')
[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.


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

An example of a catamorphism is the sum function.

sum == [not] 0 [swuncons] [+] hylomorphism
define('sum == [not] 0 [swuncons] [+] hylomorphism')
J('[5 4 3 2 1] sum')

The step combinator

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

J('[step] help')
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.
define('sum == 0 swap [+] step')
J('[5 4 3 2 1] sum')

Example: Factorial Function

For the Factorial function:

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


c == 1
F == *
G == --
P == 1 <=
define('factorial == 1 swap [1 <=] [pop] [[*] dupdip --] primrec')
J('5 factorial')

Example: tails

An example of a paramorphism for lists given in the “Bananas…” paper 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
define('tails == [] swap [not] [pop] [rest dup [swons] dip] primrec')
J('[1 2 3] tails')
[[] [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”

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

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