Tomáš Peterka

std::blogout

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Open and Closed form of Control Structure IF

When writing if prefer “closed” form instead of “open” form.

# open form
if some_condiditon:
  do_something()

# closed form
if not some_condition():
  return
do_something()

Closed form gives you clear idea which parts of code gets executed under what circumstances.

Open form

Leads naturally to deeper code which is unreadable for ordinary humans like me. Imagine more conditioned example. Are you able to state what are necessary conditions for do_C and do_D to run?

# deep open form
if condition_A:
  do_A()
  if condition_B:
    do_B()
  else:
    if condition_C:
      do_C()
  do_D()

Closed form

Encourages flat structure and returning / breaking the code early. I decided to repeat do_D() in the last two ifs for the sake of readability. Once you extract code in a function it is not a crime to call the function on many places. That is why you extract functionality into functions.

if not condition_A:
  return
do_A()

if condition_B:
  do_B()
  do_D()
  return

if condition_C:
  do_C()
  do_D()
  return

Compare again your ability to state under which conditions do_C() and do_D() run.

Some rules of thumb how to spot open forms

1. Your IFs are being folded too much
2. You have many (any) ELSEs
3. When I Wrote It, Only God and I Knew the Meaning; Now God Alone Knows