Re: [Bug-gnulib] addition: c-ctype.h, c-ctype.c

[Bruno Haible]

Paul Eggert writes:

> > however it's not a problem for the c_* functions.
> 
> I agree, but for Japanese it works only because "<yen>" happens to
> fall into the same ctype category as "\", and similarly for
> "<overbar>" and "~".  Luckily for us, the other ISO 646 national
> variants are no longer used, so we don't have to worry about the fact
> that, for instance, ISO 646-DE substituted an alphabetic character for
> the non-alphabetic ASCII character "~".  If c_ctype had been written
> 20 years ago, this would have been a real problem, but nowadays I
> think it's OK to ignore

I agree.

> No, I didn't know that.  But don't you have to parenthesize the C code
> properly?
> 
>        ((x - 1) & (- x - 1)) + 1 == (x & -x)

Yes sure.

> My own favorite property in this area is much simpler.  I learned it
> by attending a talk by Edsger W. Dijkstra, where he spent way too much
> time on it since he thought it was neat too.  The property is that
> "==" is associative on booleans.

But this is trivial: just map the booleans { true, false } to the bits
Z/2Z = { 0, 1 }, by mapping true to 0 and false to 1 (!), then "==" is
the same as "xor" or "+", and the equality of  x+(y+z)  and  (x+y)+z
is inherited from "+" on Z.

Bruno