Recursion

Warning: This article is a work in progress! No refunds if it moves!

TODO: This article is about tail calls

- introduction/overview

- 'magic of recursion'?

- lua

Recursion

As a quick refresher, recursion is when code calls itself.

Here's a textbook example:

function fac(n)
  if n < 1 then
    return 1
  else
    return n * fac(n - 1)
  end
end

print(fac(20))
-- prints 2432902008176640000

This code has a function 'fac' that takes a number and:

• Returns 1 if the number is less than 1
• Calls fac with the number minus 1
• Multiplies the result of fac by the number
• Returns the multiplied result

Unfortunately there's a problem with this. When we call fac we need to save our current number so we can multiply it with the result of fac. If we recurse too many times we run out of memory on our stack to save our numbers.

Because of this tendency to overflow the stack recursion isn't seen much in mainstream programming.

- mention stack overflow

- imagine an infinite stack

Loops

- recursion can implement loops!

- implementing a for loop

- implementing a while loop

- implementing a do while loop

- each loop iteration only shares global and function args

State machines

- implementing a stateful algorithm

- some kind of menu system

- the code makes sense to read

- this is mutual recursion

- very hard to do in a traditional structured language

Lambdas

- lambdas to actually replace looping constructs/switch statements

- most mainstream languages support lambas

- recursion-based control flow

Tail call elimination

- floating back down to reality

- we've been writing code as there's no stack

- tail call elimination

- NOT an optimization, how many optimizations decide which way you can program?

- hints deeper at function calls vs jumps

- structured programming, goto wars

Mainstream support

- functional programming languages

- lua

- clang mustcall

- webassembly