The functional while 1 loop

Posted on January 13, 2020 by Simon Plakolb
Invention of the century just short of Haskell's forM\_

Okay, this one is short and stupidly simple. As some might know, I like to come up with Haskell-ish functional solutions for problems nobody ever had in Python.

Today: How to get rid of the while loop

Say you want to download some data from an infamously inconsistent server. Downloading, processing and storing the data is all done within one process called proc. However, you already know that 99% of the time, it will leave you with a timeout. So you write a while loop which executes the request until it is successful. It looks something like this:

        while 1:
            if result_there():

or maybe, if you’re a little more tidy:

        while not success:
            success = proc()

or, if you’re really lazy in writing code:

        while not proc():

But what if I tell you, you can win the local “who used the least lines of code”-competition with a simple trick?

But first: Some hatred for the while loop

…and an explanation why we don’t even need a substitute for it.

Pure functional programming incorporates no side effects. This means there is no variables, there is no mutable state, there is no I/O. Since there is no mutable state, there is no for loops. The whole concept of a for loop is to alter state in place. (Hint: Use Comprehensions and Generator-Expressions instead!)

But what about the more primitive while loop, doesn’t it just evaluate a predicate? Well yes, but what’s the use case? To calculate the same result over and over again just for good measure? See when you calculate a solution in FP, you’ve got the solution. Since your function cannot depend on any external variables, you gotta change the input in order to change the result. You want to go over a list of inputs and safe all results? Use a fabulous list-comprehension instead! (Or a generator expression)

And now: Something just proven to be useless

How can we run a process over and over again until it succeeds without a while loop? Just use a wise combination of any and count (or any other infinite expression really):

        any(proc() for _ in count())

Why does this work? any (and all) are shortcut. This means, in order to save some computation time, they will stop iterating once they’ve:

  • any: Come across the value True
  • all: Come across the value False

Hence, there’s also a very similar approach to substitute a loop executing all processes until one fails:

        all(proc() for _ in count())

And probably there’s many other applications one can come up with. One important side note though: If you find yourself wondering why only half of your processes in your static blog software were ever executed - Maybe you’ve used an any to check if one process found something worth of updating and any shortcut out of executing the rest ;)