On Sun, 01 Jul 2012 13:48:04 +1000, Chris Angelico wrote: > On Sun, Jul 1, 2012 at 1:23 PM, Steven D'Aprano > <steve+comp.lang.pyt...@pearwood.info> wrote: >> All the worse for those languages, since they violate the semantics of >> mathematical notation. > > Not so. It simply means that booleans are nothing special. In REXX, > there are no data types at all, and "1" and "0" are your booleans. In C, > boolean and comparison operations return integers, either 1 or 0.
This has nothing to do with the presence of a Boolean type or not. It is about syntax, not types. Python didn't have bools until relatively recently but it still had chained comparisons. In Python2.1 and older, boolean and comparison operations return integers, either 1 or 0, precisely the same as C. You can even use chained comparisons for types that don't interpret the operators as comparisons: py> class Funny: ... def __init__(self, x): ... self.x = x ... def __lt__(self, other): ... return Funny(self.x + 3*other.x) ... def __str__(self): ... return str(self.x) ... py> a = Funny(2) py> b = Funny(3) py> c = Funny(4) py> py> print a < b < c 15 py> print (a < b) and (b < c) 15 Although the interpretation of such may not be useful. > Same > was true of Python early on, if I understand correctly. It's not > shameful. The first public release of Python, 0.9, included chained comparisons. This was even before the language had == for equality tests! steve@runes:~$ ./python0.9.1 >>> print 2 < 3 < 4 1 >>> print 2 = 2 = 2 1 >>> print (2 = 2) = 2 0 So no, Python has always included chained comparisons, and yes, it is shameful that a language would force you to unlearn standard notation in favour of a foolish consistency with other operators. Comparisons aren't special because they return bools. They are special because of the way they are used. C treats comparison operators as if they were arithmetic operators, and so the behaviour of a chained comparison is the same as the behaviour as a sequence of arithmetic operators: a foolish consistency. Python treats comparison operators as comparison operators, and gives them behaviour appropriate to comparisons. The fact that so many languages do the wrong thing here, and so few emulate standard mathematical notation, is symptomatic of the lousy state of language design. Pascal might be verbose, but at least it makes it harder to write code that silently does the wrong thing -- it prevents you from writing chained comparisons which do the wrong thing, and forces you to be explicit. C has the worst of both worlds: - it allows the standard concise mathematical notation a < x < b - but it quietly changes the semantics to something that the user hardly ever wants thus giving the maximum number of bugs with the minimum amount of convenience. >> The more I learn about C, the less I want to know about C. What sort of >> crazy language designer thought that having >> >> 2 == 2 == 2 >> >> return 0 (false) was a good idea? > > It makes perfect sense though; Not in English-speaking countries with a culture of writing chained comparisons in mathematics and allowing them in natural language: "Rock is beaten by Paper, is beaten by Scissors". -- Steven -- http://mail.python.org/mailman/listinfo/python-list
