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¶
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
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”
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 dequotec
. - 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
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
is0
[G]
is[-- dup]
[F]
is[+]
define('triangular_number == [1 <=] 0 [-- dup] [+] hylomorphism')
Let’s try it:
J('5 triangular_number')
10
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
¶
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
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.
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
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')
15
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')
15
Example: Factorial Function¶
For the Factorial function:
H4 == c swap [P] [pop] [[F] dupdip G] primrec
With:
c == 1
F == *
G == --
P == 1 <=
define('factorial == 1 swap [1 <=] [pop] [[*] dupdip --] primrec')
J('5 factorial')
120
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.”