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digitalmars.D - If !in is inconsistent because of bool/pointer, then so is !

reply downs <default_357-line yahoo.de> writes:
This has been brought up before as an argument against the !in operator
(forcing us to resort to such workarounds as /notin/): that the !in operator
would have inconsistent syntax with in, because in returns a pointer and !in
would return a bool.

This is NOT a reason against !in. In fact, this so-called "inconsistency" is
already present in the language. If we remember, !pointer already transforms it
into a boolean, so it would actually be more consistent if !in changed the
return type to bool.

Furthermore, many newcomers expect !in to work because it is intuitive.
Violating such user expectations should be avoided wherever possible.
Feb 06 2009
next sibling parent bearophile <bearophileHUGS lycos.com> writes:
downs:

 Furthermore, many newcomers expect !in to work because it is intuitive.
Violating such user expectations should be avoided wherever possible.



If the pressure coming from such silly limits/warts gets large enough, LDC may
start fixing them independently from DMD (I don't know what LDC developers
think about this idea).

Bye,
bearophile
Feb 06 2009
prev sibling next sibling parent grauzone <none example.net> writes:
vote++ for !in

It's one of those things, which are very annoying to programmers, but 
very easy to fix. Just do it, Walter.
Feb 06 2009
prev sibling next sibling parent Moritz Warning <moritzwarning web.de> writes:
On Fri, 06 Feb 2009 12:42:30 +0100, downs wrote:


 This has been brought up before as an argument against the !in operator
 (forcing us to resort to such workarounds as /notin/): that the !in
 operator would have inconsistent syntax with in, because in returns a
 pointer and !in would return a bool.
 
 This is NOT a reason against !in. In fact, this so-called
 "inconsistency" is already present in the language. If we remember,
 !pointer already transforms it into a boolean, so it would actually be
 more consistent if !in changed the return type to bool.
 
 Furthermore, many newcomers expect !in to work because it is intuitive.
 Violating such user expectations should be avoided wherever possible.



Having such operator/syntactic sugar would be very nice.
None of the arguments against it have convinced me so far.
Feb 06 2009
prev sibling parent reply Rainer Deyke <rainerd eldwood.com> writes:
downs wrote:

 This is NOT a reason against !in. In fact, this so-called
 "inconsistency" is already present in the language. If we remember,
 !pointer already transforms it into a boolean, so it would actually
 be more consistent if !in changed the return type to bool.



I agree.  'a != b' is short for '!(a == b)'.  'a !is b' is short for
'!(a in b)'.  For consistency, 'a !in b ' should be short for
'!(a in b)'.  I'd even go so far as to say that 'a !+ b' should be short
for '!(a + b)', although I can't think of a use for the '!+' operator.

a !<op> b == !(a <op> b): simple, consistent pattern.
a !<op> b == !(a <op> b), but only for <op> in some limited set that
doesn't include all operators with which you might want to use the
pattern: less consistent; requires memorization.


-- 
Rainer Deyke - rainerd eldwood.com
Feb 06 2009
next sibling parent reply downs <default_357-line yahoo.de> writes:
Rainer Deyke wrote:

 downs wrote:

 This is NOT a reason against !in. In fact, this so-called
 "inconsistency" is already present in the language. If we remember,
 !pointer already transforms it into a boolean, so it would actually
 be more consistent if !in changed the return type to bool.



 I agree.  'a != b' is short for '!(a == b)'.  'a !is b' is short for
 '!(a in b)'.  For consistency, 'a !in b ' should be short for
 '!(a in b)'.  I'd even go so far as to say that 'a !+ b' should be short
 for '!(a + b)', although I can't think of a use for the '!+' operator.
 
 a !<op> b == !(a <op> b): simple, consistent pattern.
 a !<op> b == !(a <op> b), but only for <op> in some limited set that
 doesn't include all operators with which you might want to use the
 pattern: less consistent; requires memorization.
 
 



Hmm ...

A large part of the case for !in is that you can pronounce it "a *not in* b".
!+, on the other hand, would be .. what? "a not plus b? does that mean a - b? "
:)
Feb 06 2009
parent reply Rainer Deyke <rainerd eldwood.com> writes:
downs wrote:

 A large part of the case for !in is that you can pronounce it "a *not
 in* b". !+, on the other hand, would be .. what? "a not plus b? does
 that mean a - b? " :)



It's a question of consistent patterns versus special cases.  If
'a !<op> b == !(a <op> b)', then the parser can rewrite all 'a !<op> b'
expressions as '!(a <op> b)' in a single place, without looking at what
<op> is.

(Of course '!=' (as the opposite of '==' as opposed to '=') is already a
special case, so perhaps defining the '!<op>' operators individually is
unavoidable.  'a !== b' as '!(a == b)' would work, but 'a != b' as '!(a
= b)' would be very weird and inconsistent with other languages.)

I'm not suggesting that anybody should actually /use/ the '!+' operator,
even if it was defined.  That would be horrible.


-- 
Rainer Deyke - rainerd eldwood.com
Feb 06 2009
parent reply bearophile <bearophileHUGS lycos.com> writes:
Rainer Deyke:

 It's a question of consistent patterns versus special cases.



You may think that for humans it's better to have a very orthogonal language,
like for example Scheme.
There's also a famous quote about this, "Programming languages should be
designed not by piling feature on top of feature, but by removing the
weaknesses and restrictions that make additional features appear necessary."
But in practice the large part of programmers work with languages like Java, C,
C++, C#, Python, modern basic variants, etc despite they have much more
restrictions compared to Scheme.
This is a long thing to explain, and I don't have enough space in this tight
post to explain it, but the short version is that removing "special cases" as
allowing !+ makes the language worse, less easy to use, more bug-prone, and
generally less good.

Bye,
bearophile
Feb 06 2009
next sibling parent bearophile <bearophileHUGS lycos.com> writes:
Bill Baxter:

 (a !in b) should exist and it should be the same as !(a in b).



Of course.

Bye,
bearophile
Feb 06 2009
prev sibling next sibling parent reply Rainer Deyke <rainerd eldwood.com> writes:
Bill Baxter wrote:

 Note that D already has things like !>.   But quoth the spec:
 "For floating point comparison operators, (a !op b)  is *NOT* the same
 as !(a op b)."
 [emphasis added]



I had to check the spec for the difference.  'a !< b' and '!(a < b)'
/are/ equivalent in the sense that '(a !< b) == !(a < b)' for any values
of 'a' and 'b'.  The vast majority of the time, the expressions 'a !< b'
and '!(a < b)' /are/ interchangeable.  The difference is that '!(a < b)'
sets a global exception state if either operand is NaN, while 'a !< b'
does not.

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)'.


-- 
Rainer Deyke - rainerd eldwood.com
Feb 06 2009
parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
Rainer Deyke wrote:

 Bill Baxter wrote:

 Note that D already has things like !>.   But quoth the spec:
 "For floating point comparison operators, (a !op b)  is *NOT* the same
 as !(a op b)."
 [emphasis added]



 I had to check the spec for the difference.  'a !< b' and '!(a < b)'
 /are/ equivalent in the sense that '(a !< b) == !(a < b)' for any values
 of 'a' and 'b'.  The vast majority of the time, the expressions 'a !< b'
 and '!(a < b)' /are/ interchangeable.  The difference is that '!(a < b)'
 sets a global exception state if either operand is NaN, while 'a !< b'
 does not.
 
 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.

  -- Daniel
Feb 06 2009
next sibling parent Don <nospam nospam.com> writes:
Daniel Keep wrote:

 
 Rainer Deyke wrote:

 Bill Baxter wrote:

 Note that D already has things like !>.   But quoth the spec:
 "For floating point comparison operators, (a !op b)  is *NOT* the same
 as !(a op b)."
 [emphasis added]



 /are/ equivalent in the sense that '(a !< b) == !(a < b)' for any values
 of 'a' and 'b'.  The vast majority of the time, the expressions 'a !< b'
 and '!(a < b)' /are/ interchangeable.  The difference is that '!(a < b)'
 sets a global exception state if either operand is NaN, while 'a !< b'
 does not.

 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.
 
   -- Daniel



Yes. It's the hardware. It's hard to find situations where the 
difference matters.  And this is why !<> and !<>= are the only ones of 
those operators which are actually useful.
Feb 06 2009
prev sibling parent reply Rainer Deyke <rainerd eldwood.com> writes:
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.


-- 
Rainer Deyke - rainerd eldwood.com
Feb 06 2009
parent Don <nospam nospam.com> writes:
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 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.
Feb 07 2009
prev sibling parent downs <default_357-line yahoo.de> writes:
Bill Baxter wrote:

 On Sat, Feb 7, 2009 at 8:54 AM, bearophile <bearophileHUGS lycos.com> wrote:

 Rainer Deyke:

 It's a question of consistent patterns versus special cases.



 There's also a famous quote about this, "Programming languages should be
designed not by piling feature on top of feature, but by removing the
weaknesses and restrictions that make additional features appear necessary."
But in practice the large part of programmers work with languages like Java, C,
C++, C#, Python, modern basic variants, etc despite they have much more
restrictions compared to Scheme.
 This is a long thing to explain, and I don't have enough space in this tight
post to explain it, but the short version is that removing "special cases" as
allowing !+ makes the language worse, less easy to use, more bug-prone, and
generally less good.



 Note that D already has things like !>.



Well yeah, but note that it can be pronounced "not bigger", as opposed to !+.
Feb 07 2009
prev sibling next sibling parent Bill Baxter <wbaxter gmail.com> writes:
On Sat, Feb 7, 2009 at 8:54 AM, bearophile <bearophileHUGS lycos.com> wrote=
:

 Rainer Deyke:

 It's a question of consistent patterns versus special cases.



 You may think that for humans it's better to have a very orthogonal langu=




 There's also a famous quote about this, "Programming languages should be =



esses and restrictions that make additional features appear necessary." But=
 in practice the large part of programmers work with languages like Java, C=
, C++, C#, Python, modern basic variants, etc despite they have much more r=
estrictions compared to Scheme.

 This is a long thing to explain, and I don't have enough space in this ti=



es" as allowing !+ makes the language worse, less easy to use, more bug-pro=
ne, and generally less good.

Note that D already has things like !>.   But quoth the spec:
"For floating point comparison operators, (a !op b)  is *NOT* the same
as !(a op b)."
[emphasis added]

But anyway I wholeheartedly agree that (a !in b) should exist and it
should be the same as !(a in b).

I think the principle of least surprise is generally a good one to
follow.  And I think most people are surprised that (a !is b) means
!(a is b),   while the same is not true of (a !in b).

--bb
Feb 06 2009
prev sibling parent "Simen Kjaeraas" <simen.kjaras gmail.com> writes:
On Sat, 07 Feb 2009 00:39:33 +0100, Rainer Deyke <rainerd eldwood.com> wrote:


 (Of course '!=' (as the opposite of '==' as opposed to '=') is already a
 special case, so perhaps defining the '!<op>' operators individually is
 unavoidable.  'a !== b' as '!(a == b)' would work, but 'a != b' as '!(a
 = b)' would be very weird and inconsistent with other languages.)



From the identity (a != b) == !(a == b), you could argue that
(a !<op> b) == !(a <op>= b), but I can hardly see that being better. :p


--
Simen
Feb 08 2009