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digitalmars.D - Why does D change operator precedence according to C/C++ ?

reply deadalnix <deadalnix gmail.com> writes:
Hi,

I have a design question, or maybe it is a bug ? In D, == and != have 
the same precedence than comparisons operators. This isn't the case in 
C/C++ . Is it a design decision, made on purpose ? Is it a bug ?

DMD implementation and http://dlang.org/expression.html both agree on that.

I personally think it is a bug. This is a change in the behavior of 
C/C++ with no legitimate reason. Or at least with no legitimate reason I 
can come up with.
Apr 02 2012
next sibling parent "Andrea Fontana" <nospam example.com> writes:
Probably a good way to force parentesys usage and avoid subtle 
bugs...

On Monday, 2 April 2012 at 10:01:20 UTC, deadalnix wrote:

 Hi,

 I have a design question, or maybe it is a bug ? In D, == and 
 != have the same precedence than comparisons operators. This 
 isn't the case in C/C++ . Is it a design decision, made on 
 purpose ? Is it a bug ?

 DMD implementation and http://dlang.org/expression.html both 
 agree on that.

 I personally think it is a bug. This is a change in the 
 behavior of C/C++ with no legitimate reason. Or at least with 
 no legitimate reason I can come up with.
Apr 02 2012
prev sibling parent reply "Simen Kjaeraas" <simen.kjaras gmail.com> writes:
On Mon, 02 Apr 2012 12:03:14 +0200, deadalnix <deadalnix gmail.com> wrote:


 Hi,

 I have a design question, or maybe it is a bug ? In D, == and != have  
 the same precedence than comparisons operators. This isn't the case in  
 C/C++ . Is it a design decision, made on purpose ? Is it a bug ?

 DMD implementation and http://dlang.org/expression.html both agree on  
 that.

 I personally think it is a bug. This is a change in the behavior of  
 C/C++ with no legitimate reason. Or at least with no legitimate reason I  
 can come up with.


It's there to force proper parenthezation:

1 <= 3 == 2 > 3 != 3 < 1
vs
((1 <= 3) == (2 > 3)) != (3 < 1)
vs
(1 <= (3 == 2)) > ((3 != 3) < 1)
Apr 02 2012
parent reply Stewart Gordon <smjg_1998 yahoo.com> writes:
On 02/04/2012 11:32, Simen Kjaeraas wrote:

 On Mon, 02 Apr 2012 12:03:14 +0200, deadalnix <deadalnix gmail.com> wrote:

 I have a design question, or maybe it is a bug ? In D, == and != have the same
 precedence than comparisons operators. This isn't the case in C/C++ . Is it a
design
 decision, made on purpose ? Is it a bug ?




<snip>

 It's there to force proper parenthezation:


<snip>

That combined with associativity rules.

In C(++),
     a < b == c <= d
means
     (a < b) == (c <= d)

However, if one simply made all the comparison operators equal precedence, then
the 
meaning of this would change to
     ((a < b) == c) <= d

However, what has actually been done is to make these operators non-associative
as well, 
in order to render it an illegal expression.  This was done to prevent subtle
typos or 
confusion with other languages/notations where chaining of comparison operators
denotes a 
conjunction of the comparisons.

Another way to look at it is that the different comparison operators have no
precedence 
relative to each other (though each still needs to be non-associative). 
Indeed, another 
change that has taken place is to make the comparison operators have no
precedence 
relative to the bitwise boolean operators, simply because the C precedence
rules here were 
confusing.
http://d.puremagic.com/issues/show_bug.cgi?id=4077

Stewart.
Apr 02 2012
parent reply deadalnix <deadalnix gmail.com> writes:
Le 02/04/2012 13:10, Stewart Gordon a écrit :

 On 02/04/2012 11:32, Simen Kjaeraas wrote:

 On Mon, 02 Apr 2012 12:03:14 +0200, deadalnix <deadalnix gmail.com>
 wrote:

 I have a design question, or maybe it is a bug ? In D, == and != have
 the same
 precedence than comparisons operators. This isn't the case in C/C++ .
 Is it a design
 decision, made on purpose ? Is it a bug ?




 <snip>

 It's there to force proper parenthezation:


 <snip>

 That combined with associativity rules.

 In C(++),
 a < b == c <= d
 means
 (a < b) == (c <= d)

 However, if one simply made all the comparison operators equal
 precedence, then the meaning of this would change to
 ((a < b) == c) <= d

 However, what has actually been done is to make these operators
 non-associative as well, in order to render it an illegal expression.
 This was done to prevent subtle typos or confusion with other
 languages/notations where chaining of comparison operators denotes a
 conjunction of the comparisons.

 Another way to look at it is that the different comparison operators
 have no precedence relative to each other (though each still needs to be
 non-associative). Indeed, another change that has taken place is to make
 the comparison operators have no precedence relative to the bitwise
 boolean operators, simply because the C precedence rules here were
 confusing.
 http://d.puremagic.com/issues/show_bug.cgi?id=4077

 Stewart.


So basically, the precedence doesn't matter because any situation where 
it matter is illegal anyway ?
Apr 02 2012
next sibling parent "Andrea Fontana" <nospam example.com> writes:
Parentesys are needed only where there's ambiguity with 
precedence.

a > b > c is ambiguous (we need parentesys)
a == b > c  is amibiguous (we need parentesys because == and > 
have the same precedence)

a == b && c is not ambiguous ( && and == haven't the same 
precedence so we don't need parentesys)

On Monday, 2 April 2012 at 12:02:33 UTC, deadalnix wrote:

 Le 02/04/2012 13:10, Stewart Gordon a écrit :
 So basically, the precedence doesn't matter because any 
 situation where it matter is illegal anyway ?
Apr 02 2012
prev sibling parent reply Walter Bright <newshound2 digitalmars.com> writes:
On 4/2/2012 5:04 AM, deadalnix wrote:

 So basically, the precedence doesn't matter because any situation where it
 matter is illegal anyway ?


That's correct and neatly sums up the situation.
Apr 02 2012
parent deadalnix <deadalnix gmail.com> writes:
Le 02/04/2012 15:49, Walter Bright a écrit :

 On 4/2/2012 5:04 AM, deadalnix wrote:

 So basically, the precedence doesn't matter because any situation
 where it
 matter is illegal anyway ?


 That's correct and neatly sums up the situation.


Merci :D
Apr 02 2012