If !in is inconsistent because of bool/pointer, then so is !

[Don]

Rainer Deyke wrote:
> Daniel Keep wrote:
>> Rainer Deyke wrote:
>>> This is, in my opinion, a significant design error in the language.  The
>>> difference between '!(a < b)' and 'a !< b' is not obvious.  There is
>>> nothing about the operator '<' that suggests that it should set a global
>>> exception state, and there is nothing about '!<' that suggests that it
>>> should /not/ set a global exception state.  (Is global state for error
>>> reporting ever a good idea in a high-level language?)  It also adds
>>> awkward expressions to the language, not just in the form '!(a < b)',
>>> but in the form '!(a !< b)'.
>> I believe this is, or is the result of, an aspect of IEEE floating point.
> 
> I don't have a copy of the IEEE floating point standard, but I strongly
> suspect it would have allowed syntax like:
>   a < b // sets global state
>   a !< b // sets global state
>   less_than_no_state(a, b) // does not set global state
>   !less_than_no_state(a, b) // does not set global state
> Or:
>   a < b // does not set global state
>   a !< b // does not set global state
>   less_than_set_state(a, b) // sets global state
>   !less_than_set_state(a, b) // sets global state
> Or:
>   a < b // sets global state
>   a !< b // sets global state
>   a [<] b // does not set global state
>   a [!<] b // does not set global state
> Or any number of other syntax choices, all less confusing than the
> syntax actually used.
> 
> This is assuming we need two sets of comparison operators, one of which
> uses global state to report NaN operands and one which does not.  I'm
> not convinced that this is the case.

I don't have the final standard, but the last publically avilable draft 
(IEE754R Draft 1.2.5, Oct 2006) states:

"The unordered-signaling predicates in Table 9, intended for use by 
programs not written to take into account
the possibility of NaN operands, signal an invalid exception on quiet 
NaN operands:" [table 9]
"The unordered-quiet predicates in Table 10, intended for use by 
programs written to take into account the
possibility of NaN operands, do not signal an exception on quiet NaN 
operands:" [table 10]

"There are two ways to write the logical negation of a predicate, one 
using NOT explicitly and the other
reversing the relational operator. Thus in programs written without 
considering the possibility of a NaN
operand, the logical negation of the unordered-signaling predicate (X < 
Y) is just the unordered-signaling
predicate NOT(X < Y); the unordered-quiet reversed predicate (X ?>= Y) 
is different in that it does not signal
an invalid operation exception when X and Y are unordered. In contrast, 
the logical negation of (X = Y) may
be written either NOT(X = Y) or (X ?<> Y); in this case both expressions 
are functionally equivalent to (X != Y).

The "global state" you're referring to is just a flag in the 
floating-point status register.

Although I believe you could implement these comparisons as functions 
rather than operators, they'd have to be intrinsics. On x87, the normal 
comparisons are done with the FCOM instruction, and the others are done 
with FCOMI. That's the only difference.