Why if needs to be an expression

0_if_expr.ml

let result = match is_correct some_value with
  | true -> some_result_function 42
  | false -> some_error_function 23
in
(* use result here, no mutation, generalizes to n cases easily *)

1_if_stmt.py

result = None
if is_correct(some_value):
  result = some_result_function(42)
else:
  result = some_error_function(23)

# use result here
# but now you have mutation
# also, what happens if you forget to set result in one of the branches?

2_if_ternary.py

result = some_result_function(42) if is_correct(some_value) else some_error_function(23)

# use result here
# but now it is hard to read, the condition is in the middle
# and if you have more than simple code in your branches the code will get even worse 
# also, it only works for if/else, but not for if/elseif/elseif/elseif/else

3_if_expr.py

def _inner(condition):
  if condition:
    return some_result_function(42)
  else:
    return some_error_function(23)
  
result = _inner(is_correct(some_value))

# use result here
# no mutation
# but now every time you want to use if/then/else you need to write it as a function with some meaningless name
# and then call it
# what happens if you forget to return in one of the branches?