The lil' serpent who bites its own tail

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Recursion, snakes and tail call optimization

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My       with recursion

A heartbreaking yet real story

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❤️

"But it is very difficult to grasp"

"to understand recursion you first need to understand recursion"

Recursion starter pack

Factorial!

What is recursion anyway?

📖 "Recursion occurs when a thing is defined in terms of itself or of its type. [...] While this apparently defines an infinite number of instances , it is often done in such a way that no infinite loop occurs" 📖

Looking for a file "example.py"

      📂 /jluna

       📄 InTheBeach.png.exe

              📂 /Downloads

📂 /home

      📄 passwrd.txt

      📂 /jluna

     📂 /Downloads

       🐍 example.py

Binary search: Is '4' here or what?

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

[0, 1, 2, 3, 4]

[3, 4]

Nope, but 4 < 5

Nope, but 4 > 2

Oh yeah!!

Types of recursion

  • Simple: Single reference to itself
     
  • Compound: Several references to itself

 

  • Indirect: Calls a function that calls the initial function

 

  • Anonymous: Slight variation from simple/compound but with anonymous functions
def factorial(n):
  if n in {0, 1}:
    return 1
  return n * factorial(n-1) <---

def fib(n):
  if n in {0, 1}:
    return n
  return fib(n-1) + fib(n-2) <-- 2x
###
def a():
  b() <---
  
def b():
  a() <---
###  
  
def f(callable):
  callable() <---

Looking closely...

  • Solutions to those problems that "feel more natural" are recursive. Why?

 

  • It is the data structure that suggests a recursive algorithm or not

And which type of recursion does Python "allow me to use"?

...
def f9996():
  return f9995()
def f9997():
  return f9996()
def f9998():
  return f9997()
def f9999():
  return f9998()


f9999()

😭

  File "recursive.py", line 18004, in f9001
    return f9000()
RecursionError: maximum recursion depth exceeded

Enter...

"The Stack"

f_a(1,2)

f_b()

f_c(42)

f_d("hello!")

The Call Stack

__main__()

def fun_a():
  return "Hello!"

def fun_b():
  return fun_a()

fun_b() <---

__main__() -> fun_b()

__main__

def fun_a():
  return "Hello!"

def fun_b(): <---
  return fun_a()  <---

fun_b()

__main__()

       └──fun_b() -> fun_a()

 

fun_b()

The Call Stack

__main__

def fun_a(): <---
  return "Hello!" <---

def fun_b():
  return fun_a()

fun_b()

__main__()

       └──fun_b()

                      └── fun_a() -> "Hello!"

 

fun_b()

fun_a()

The Call Stack

__main__

def fun_a():
  return "Hello!"

def fun_b():
  return fun_a() <--- "Hello!"

fun_b()

__main__()

       └──fun_b() -> "Hello!"

fun_b()

The Call Stack

__main__

def fun_a():
  return "Hello!"

def fun_b():
  return fun_a()

fun_b() <--- "Hello!"

__main__() -> "Hello!"

The Call Stack

Stack frames

def f_a(arg1, arg2):
  local_var = 3
  return f_b(3, 4)

Loaded variables:

arg1, arg2, local_var, f_b

Code itself

Book-keeping:

file name, row #, reference to past stack frames..

function(3, 4)

import inspect

import inspect

stack = inspect.stack()

current_frame = stack[0]

loaded_vars = current_frame.frame.f_locals

print(loaded_vars)

RecursionError, a soft StackOverflow error

f_a(1,2)

f_b()

f_c(42)

f_d("hello!")

The stack has limits

Logical limit

Physical limit

f_a(1,2)

f_b()

f_c(42)

f_d("hello!")

The stack has limits

The value of the logical limit depends on the system, but is usually capped to 1000

import sys

sys.getrecursionlimit() -> 1000

sys.setrecursionlimit(100_000_000)

f_a(1,2)

f_b()

f_c(42)

f_d("hello!")

RecursionError!

Logical limit

Physical limit

f_d("hello!")

Optimising tail calls 🐍

(or...not *cries recursively*)

What on earth is a tail call?

A call to a subroutine is "in the tail" if it is the last statement of a procedure

def f():
  return a()

def f2(n):
  if n == 1:
    return a()
  else:
    return b()
def f():
  result = a()
  print(result)
  return result

def f2(n):
  return a() + 1

✅

❌

Why are they special?

  • Stack frames need to be in the stack as long as they are necessary
     
  • If calls to a function are done in the tail (last statement), we can foresee that the stack frame won't be needed anymore.
     
  • Could we delete the stack frame then?

f_a(1,2)

f_b()

f_c(42)

f_d("hello!")

Tail Call Optimization

  • Delete frames maybe not... but we can reuse them!
  • That way we save on the overhead needed to create a new stack frame

a()

└─b() -> c()

a()

b()

a()

└─ c()

a()

c()

a()

a() "wtf I called b() not c()"

Let's do it then, what are we waiting for?

😀

 

"Cuz tracebacks from exceptions would be hella confusing then"

Tracebacks

Traceback (most recent call last):
  File "illooo.py", line 10, in <module>
    c()
  File "illooo.py", line 8, in c
    b()
  File "illooo.py", line 5, in b
    a()
  File "illooo.py", line 2, in a
    raise Exception("Obscure exception")
Exception: Obscure exception
Traceback (most recent call last):
  File "illooo.py", line 5, in <module>
    a()
  File "illooo.py", line 2, in a
    raise Exception("Obscure exception")
Exception: Obscure exception

Con TCO

Without TCO

Now what?

You cannot top a good ol' loop...

def fib(n):
 if n in {0, 1}:
  return n
 return fib(n-1) + fib(n-2)
def fib(n):
  if n in {0, 1}:
    return n
  for _ in range(n):
    a, b = b, a + b
  return a
  • In imperative languages, subroutines calls cause a lot of overhead
     
  • "For each recursive implementation there's an equivalent iterative implementation"

  • If the iterative solution is easier to understand, it is preferable to use it
    • If not... 🤷‍♂️🤷‍♀️

Reality check

Python is slow

Python is slow

  • CPython doesn't have any type of JIT
    • Tradeoff flexibility/clarity vs performance
    • Just use PyPy smh
  • What is slow is calling functions, not recursion itself
  • Python allows to interoperate with code written in C

Although in most cases...

It is "fast enough"

Any questions?

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Thankssssss for attending!

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La culebrilla que se muerde la cola

By Javier Luna Molina

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