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digitalmars.D.bugs - [Issue 7301] New: RegexMatch opCast!bool not working

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           Summary: RegexMatch opCast!bool not working
           Product: D
           Version: D2
          Platform: x86_64
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: jlquinn optonline.net


--- Comment #0 from Jerry Quinn <jlquinn optonline.net> 2012-01-16 19:12:04 PST
---
Ubuntu 11.10 x86_64, dmd 2.057

bug2.d contains:

import std.regex;
bool foo() {
  auto re = regex("test");
  return match("test", re);
}

~/dmd2/linux/bin32/dmd -c bug2.d
bug2.d(4): Error: cannot implicitly convert expression (match("test",re)) of
type RegexMatch!(string,ThompsonMatcher) to bool

It works if I use match("test", re).empty

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--- Comment #1 from Jerry Quinn <jlquinn optonline.net> 2012-01-16 19:12:59 PST
---
Actually I think this might be a DMD bug.

!match() compiles too.

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timon.gehr gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |timon.gehr gmx.ch


--- Comment #2 from timon.gehr gmx.ch 2012-01-16 19:40:52 PST ---
This is the same for basic types.

bool foo(){return 2;}  // error
bool bar(){return !2;} // fine

I don't think we are looking at a bug here.

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--- Comment #3 from Jerry Quinn <jlquinn optonline.net> 2012-01-16 21:34:51 PST
---
(In reply to comment #2)

 This is the same for basic types.
 
 bool foo(){return 2;}  // error
 bool bar(){return !2;} // fine
 
 I don't think we are looking at a bug here.


Are you saying that type conversion isn't supposed to happen when a value is
returned?

http://www.d-programming-language.org/statement.html#ReturnStatement says:

"The Ex­pres­sion is im­plic­itly con­verted to the func­tion re­turn type."

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--- Comment #4 from timon.gehr gmx.ch 2012-01-16 23:33:27 PST ---
Neither int (outside the range [0,1]) nor RegexMatch implicitly convert to
bool. Negating counts as an explicit cast to bool.

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--- Comment #5 from Jerry Quinn <jlquinn optonline.net> 2012-01-17 06:42:58 PST
---
(In reply to comment #4)

 Neither int (outside the range [0,1]) nor RegexMatch implicitly convert to
 bool. Negating counts as an explicit cast to bool.


As far as I can tell, RegexMatch does implicitly cast to bool.

struct RegexMatch {
  T opCast(T : bool);
}

I could be missing something, but this effectively declares an opCast!bool
operator.

http://www.d-programming-language.org/operatoroverloading.html#Cast claims that
both ! and bare references will get rewritten to opCast!bool:

  if (e)   =>  if (e.opCast!(bool))
  if (!e)  =>  if (!e.opCast!(bool))

That should take care of the return, no?

Note that the same compile failure happens if I write:

  auto re = regex("test");
  if (match("test", re)) { ... }


It strikes me that the language docs are rather sparse on defining what can and
cannot be implicitly converted.

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--- Comment #6 from timon.gehr gmx.ch 2012-01-17 10:19:29 PST ---
(In reply to comment #5)

 (In reply to comment #4)

 Neither int (outside the range [0,1]) nor RegexMatch implicitly convert to
 bool. Negating counts as an explicit cast to bool.


 
 As far as I can tell, RegexMatch does implicitly cast to bool.
 
 struct RegexMatch {
   T opCast(T : bool);
 }
 
 I could be missing something, but this effectively declares an opCast!bool
 operator.
 


opCast means explicit cast, as in cast(bool)regexMatch or !regexMatch or
regexMatch && 2.


 http://www.d-programming-language.org/operatoroverloading.html#Cast claims that
 both ! and bare references will get rewritten to opCast!bool:
 
   if (e)   =>  if (e.opCast!(bool))
   if (!e)  =>  if (!e.opCast!(bool))
 
 That should take care of the return, no?


That only works for conditions and arguments to boolean operators. If you think
it should also work for return values where the enclosing function is declared
to return bool, then that is an enhancement request. It might be a reasonable
one.


 
 Note that the same compile failure happens if I write:
 
   auto re = regex("test");
   if (match("test", re)) { ... }
 


Works for me.


 
 It strikes me that the language docs are rather sparse on defining what can and
 cannot be implicitly converted.


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--- Comment #7 from Jerry Quinn <jlquinn optonline.net> 2012-01-17 12:57:07 PST
---
(In reply to comment #6)

 (In reply to comment #5)

 (In reply to comment #4)

 Neither int (outside the range [0,1]) nor RegexMatch implicitly convert to
 bool. Negating counts as an explicit cast to bool.


 


 opCast means explicit cast, as in cast(bool)regexMatch or !regexMatch or
 regexMatch && 2.


I think this is the crux of what we're arguing about.  To me, explicit casting
happens when you write cast(T) in the code.  When the compiler generates a cast
for you, that's implicit casting.

Is this not correct?

It seems to me that opCast is pretty useless unless the compiler can decide to
use it to convert an object for you.


 http://www.d-programming-language.org/operatoroverloading.html#Cast claims that
 both ! and bare references will get rewritten to opCast!bool:
 
   if (e)   =>  if (e.opCast!(bool))
   if (!e)  =>  if (!e.opCast!(bool))
 
 That should take care of the return, no?


 
 That only works for conditions and arguments to boolean operators. If you think
 it should also work for return values where the enclosing function is declared
 to return bool, then that is an enhancement request. It might be a reasonable
 one.


Why should it only work in those cases?  I can't find anything in the language
docs that says this.  The docs on functions say that when a type is passed in,
implicit conversion is used as one of the match possibilities.  That's opCast,
no?  So if parameter passing is using opCast for implicit conversion, the
return statement should be too.  Hence my labeling this as a bug.


 Note that the same compile failure happens if I write:
 
   auto re = regex("test");
   if (match("test", re)) { ... }
 


 
 Works for me.


My mistake - this case does work.

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--- Comment #8 from timon.gehr gmx.ch 2012-01-17 13:13:19 PST ---
(In reply to comment #7)

 (In reply to comment #6)

 (In reply to comment #5)

 (In reply to comment #4)

 Neither int (outside the range [0,1]) nor RegexMatch implicitly convert to
 bool. Negating counts as an explicit cast to bool.


 


 opCast means explicit cast, as in cast(bool)regexMatch or !regexMatch or
 regexMatch && 2.


 
 I think this is the crux of what we're arguing about.  To me, explicit casting
 happens when you write cast(T) in the code.  When the compiler generates a cast
 for you, that's implicit casting.
 
 Is this not correct?


In D, cast(T) is an explicit cast as is using an object in a boolean context
such as if(...) !... ...||... ...&&... etc. Those are presumably the cases
referred to on the Operator Overloading page.


 
 It seems to me that opCast is pretty useless unless the compiler can decide to
 use it to convert an object for you.
 


It cannot, except in the cases where it inserts cast(bool).


 http://www.d-programming-language.org/operatoroverloading.html#Cast claims that
 both ! and bare references will get rewritten to opCast!bool:
 
   if (e)   =>  if (e.opCast!(bool))
   if (!e)  =>  if (!e.opCast!(bool))
 
 That should take care of the return, no?


 
 That only works for conditions and arguments to boolean operators. If you think
 it should also work for return values where the enclosing function is declared
 to return bool, then that is an enhancement request. It might be a reasonable
 one.


 
 Why should it only work in those cases? 


It is about disabling some narrowing implicit conversions to bool while still
allowing them were it really makes sense.

bool b = 2; // error, int does not implicitly convert to bool
if(2){...} // fine, because explicit cast(bool)2 works.


 I can't find anything in the language docs that says this. 
 The docs on functions say that when a type is passed in,
 implicit conversion is used as one of the match possibilities.  That's opCast,
 no? 


No. opCast cannot be used for implicit conversions. At some point there was
even the idea of introducing opImplicitCast.


 So if parameter passing is using opCast for implicit conversion, the
 return statement should be too.  Hence my labeling this as a bug.
 


Parameter passing does _not_ use opCast for implicit conversions.

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