Discussion on: Typing your way into safety

[Israël Hallé]

I think to keep the same spirit, I would abstract it in another layer. I would define a function resize that takes in an int. As for specific range, NewType could also be used the same way we did for strings:

Size = NewType("Size", int)

def parse_size(unsafe_size: str) -> Size:
  size = int(size_str)
  return valid_size(size)


def valid_size(unsafe_size: int) -> Size:
  if size > 2000:
    raise ValueError("Too big!")
  return size

# int as str are always safe for shell!
def shell_int(x: int) -> ShellQuotedString:
  return str(x)

def resize(size: Size) -> None:
  command = shell_format("convert INPUT -resize {} OUTPUT", shell_int(size))
  system(command)

resize(parse_size("123")) # ok
resize(valid_size(123)) # ok
resize(123) # error!

[tlavoie]

OK, that makes sense thanks. As long as you're strict about only accepting the appropriate typed versions at the command input, enforcement works. I think what is missing is the equivalent of Haskell's enforcing of types everywhere; this is a useful tool, but you have to pay attention to make sure it's used consistently.

Having it enforced everywhere would be more of a challenge I suppose, because there isn't really the equivalent of that compile-time checking in a more dynamic environment.