Re: Integers-related trap in Lua 5.3

[Roberto Ierusalimschy <roberto@...>]

> It was thus said that the Great Roberto Ierusalimschy once stated:
> > 
> > One problem with all proposed solutions is math.mininteger. The
> > lexer reads numbers without signal. (Several languages do the
> > same, e.g. C).  This means that -9223372036854775808 is read as
> > -(9223372036854775808) (a unary minus applied over a positive
> > constant). But 9223372036854775808 is not a valid integer, so it would
> > result in a float (or nil, or inf, or error). Whatever the choice, the
> > final value of -9223372036854775808 would not be an integer, despite
> > it being a valid integer.
> 
>   I haven't looked at the code, but couldn't the lexer read the minus sign
> and set a flag. Then read in the number as an unsigned quantity and as long
> as it's not a float (no decimal point, no 'e' or 'E', etc.) then check
> against LLONG_MIN and LLONG_MAX? 9223372036854775808 *is* representable as
> an unsigned quantity.

The lexer cannot distinguish between a "-9223372036854775808" in
"x = -9223372036854775808" and in "x = x-9223372036854775808". It
handles both the same way, so the only way is a minus and then
a constant. (As I said, this is a common thing in programming
languages; C and Java, for instance, have this same rule.) There
are work arounds, but they are work arounds... Java, for instance,
has an axplicit provision for that case:

  It is a compile-time error if a decimal literal of type int is larger
  than 2147483648 (2^31), or if the decimal literal 2147483648 appears
  anywhere other than as the operand of the unary minus operator

Moreover, I don't think unsigned quantities solve the problem that
people are trying to solve here (unespected results from long
integers.) People that don't undestand what is going on don't
undertand unsigned quantities, either.

(I also think that people that don't undestand what is going on do not
write numbers like 10000000000000000000 in configuration files, so the
real problem seems more nit-picking than anything else to me, but I
may be wrong.)

-- Roberto