Question about Ada expressions

Question about Ada expressions

[Perry Schmidt]

In Ada, when building boolean expressions you must put ANDs and ORs (and
OR ELSES, etc) in parents IF you use them in mixed combination.

Ex.  (A and B or C) -- is not legal.  You must write it as...
     (A and (B or C))

So the question is -- WHY?

Does this remove some semantic ambiguity?  Or is it just the "Ada style!"

Thanks for any responses...

Perry Schmidt
(schmidt@green.cs.wisc.edu)

Re: Question about Ada expressions

[STEPHEN D. STRADER]

In article <8834@spool.cs.wisc.edu> schmidt@green.cs.wisc.edu (Perry Schmidt) writes:
>In Ada, when building boolean expressions you must put ANDs and ORs (and
>OR ELSES, etc) in parents IF you use them in mixed combination.
>Ex.  (A and B or C) -- is not legal.  You must write it as...
>     (A and (B or C))
>So the question is -- WHY?

BECAUSE!

No, actually it is to remove ANY ambiguity as to how the expression
should be evaluated.  

Saright?

S

Re: Question about Ada expressions

[Robert Firth]

In article <8834@spool.cs.wisc.edu> schmidt@green.cs.wisc.edu (Perry Schmidt) writes:

>In Ada, when building boolean expressions you must put ANDs and ORs (and
>OR ELSES, etc) in parents IF you use them in mixed combination.

>So the question is -- WHY?

There is no problem of semantic ambiguity, any more than with, say,
A + B*C.  The reason for the restriction is that one of the language
designers believed the relative priorities of the Boolean operators
were sufficiently unfamiliar to the general run of programmers that
expressions such as

	A and B or C

could cause confusion or, worse, be misinterpeted.

Re: Question about Ada expressions

[stt]

Ada tried to use the "conventional" precedence
rules when defining operators.  However, it has
never been entirely clear where XOR belongs relative
to AND and OR, nor how regular AND
and short-circuit AND THEN should
fare relative to one another.  The net result
was that they "punted" in this area and said
use parentheses when using two logical operators
in sequence.  

It might have been ideal in some
people's mind if at least AND had clearly higher precedence 
than OR so that your example could be coded without
parentheses, but Ada has always tried to err
on the side of readability over writability.

S. Tucker Taft
Intermetrics, Inc.
Cambridge, MA

Re: Question about Ada expressions

[Karl Nyberg]

I had a friend call yesterday after spending five hours trying to figure out
why some C code didn't work based on this expression:

	if (variable & MASK == VALUE)

He mistook the rules of precedence, just as Robert Firth expressed it in his
response.  The K&R book explains this reasoning:

	Note that the precedence of the bitwise logical operators &, ^ and |
	falls below == (test for equality) and !=  (test for inequality).
	This implies that bit-testing expressions like

		if ((x & MASK) == 0) ...

	must be fully parenthesized to give proper results.  [p. 49]

Seems like it was kind of the Ada designers to preclude this from the
language.  Getting an error during compile time would have saved my friend
immense difficulty.  Getting a manual might have helped him, too...

-- Karl --

Re: Question about Ada expressions

[horst]

In article <8834@spool.cs.wisc.edu> schmidt@green.cs.wisc.edu (Perry Schmidt) writes:
>
>In Ada, when building boolean expressions you must put ANDs and ORs (and
>OR ELSES, etc) in parents IF you use them in mixed combination.
>...
>Does this remove some semantic ambiguity?  Or is it just the "Ada style!"

Yes, there is an ambiguity. The precedence of 'and' and 'or'
is not so commonly agreed as that of '*' and '+'. So the
designers chose to require parenthesis.
Regards,
 horst

Re: Question about Ada expressions

[Scott Simpson]

In article <8910122337.AA06188@grebyn.com> karl@grebyn.com (Karl Nyberg) writes:
>I had a friend call yesterday after spending five hours trying to figure out
>why some C code didn't work based on this expression:
>
>	if (variable & MASK == VALUE)

Your buddy didn't use lint.  It would have caught this.
	Scott Simpson
	TRW Space and Defense Sector
	usc!trwarcadia!simpson  	(UUCP)
	trwarcadia!simpson@usc.edu	(Internet)