Re: Overloaded logic operators

On Sun, 23 Nov 2008 13:13:28 -0800, Mike Austin wrote:


> In many languages, logic operators "and" and "or" have been overloaded to
> handle more than booleans. For example:
>
> x = x or 0 # if x is nil, 0
> x and x.foo() # if x is not nil, call foo
> x = x > 0 # if x is > 0, x, else false


Is "=" an assignment or equality here? What are the association priorities
of the operations "=", ">" and "or"? What are the types involved? Do you
mean short circuit operations, which do not evaluate their arguments
eagerly?


> It enables you do to tricks like this:
>
> x = -1 < x < 1 or 0
>
> but it also means your booleans are values, and you can't test "if x == true",
> but only "if x != nil"


Can you explain this? How Booleans cannot be values? To me the operation
"!=" must yield a result. This result has a value, that value has a type,
Boolean, I suggest.


As for macro expressions -1 < x < 1 or 0, which I suppose should read:


(-1 < x) and (idem < 1) or (idem < 0)


here idem denotes the value of the expression x (evaluated strictly once),
you would need a lot of work to make such macros working. Some time ago I
played with the idea for a while, but came to the conclusion that it would
not be worth the efforts and could be very confusing.
--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
[The A < B < C syntax has been part of Cobol for many decades. It seems
to work fine there. -John]

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