https://snarky.ca/unravelling-if-statements/

Tall, Snarky Canadian

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Oct 11, 2022 1 min read syntactic sugar

Unravelling `if` statements

After the initial posting of my summary post about my syntactic sugar
series, I received the following reply to one of my tweets:

     wondering... 'if/else' (14 on your list) can be implemented in
    terms of 'while' (15 on your list)? something like this:

    run_else = True
    while condition:
    run_else = False
    ...
    break
    while run_else:
    ...
    break

    'elif' can be nested/recursive inside 2nd 'while'

    -- wouter bolsterlee  (@wbolster) August 15, 2022

Now I had already dealt with else (and elif) when it came to if, but
the tweet did make me realize that I could actually unravel if
itself!

The key insight is that if you put a break at the very end of a while
statement, it will exit the loop unconditionally. That means if you
make the conditional guard on the while loop be what the conditional
guard would have been for an if statement, you end up with the same
outcome!

Take the following example:

if A:
    B

Simple if statement example

By placing that break statement at the end of the block, you can
translate that if statement into a while statement:

while A:
    B
    break

Unravelling of if into while by using break

And since break was already unravelled, you can simply this even
further:

_done = False

while not _done and A:
    B
    _done = True

Unravelling if into while without using break

The key observation is you must put the break-substituting circuit
breaker first in the conditional guard to avoid executing the
conditional guard inherited from the if statement itself. If you
don't then you run the risk of side-effects in the if guard being
triggered twice (and that would never happen in a normal if
statement).

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