Re: [Caml-list] equality operators in OCaml
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On Friday 25 July 2008 04:14:22 pm Matthew William Cox wrote: > On Thu, Jul 24, 2008 at 09:40:36PM -0400, Peng Zang wrote: > > Yeah, that always seemed broken to me. If two things are physically > > equal (they occupy the same memory space) it doesn't make sense for them > > to be structurally unequal (contain different content). Personally, one > > of the first things I did is redefined (=) to fix this. > > It's not broken at all, but complient with a common and longstanding > idiom (at least amoung scientists working with numerical codes.) The way > we test for NaN is by comparing to itself, eg: > > if x = x then (* x is a number *) else (* x is NaN *) > > Matt Yeah, I just learned that today. But these days, most languages have functions for testing for NaN. OCaml has classify_float for example. Just FYI Peng -BEGIN PGP SIGNATURE- Version: GnuPG v2.0.7 (GNU/Linux) iD8DBQFIilNnfIRcEFL/JewRAiRjAJ9jRQy37/JtI+nZ9fhiInp3HOvKWgCgtmIv k2koED9DunAM73PmaEPhnbg= =SaDz -END PGP SIGNATURE- ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] equality operators in OCaml
On Thu, Jul 24, 2008 at 09:40:36PM -0400, Peng Zang wrote: > Yeah, that always seemed broken to me. If two things are physically equal > (they occupy the same memory space) it doesn't make sense for them to be > structurally unequal (contain different content). Personally, one of the > first things I did is redefined (=) to fix this. It's not broken at all, but complient with a common and longstanding idiom (at least amoung scientists working with numerical codes.) The way we test for NaN is by comparing to itself, eg: if x = x then (* x is a number *) else (* x is NaN *) Matt signature.asc Description: Digital signature ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] equality operators in OCaml
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On Friday 25 July 2008 06:34:54 am Christophe TROESTLER wrote: > On Thu, 24 Jul 2008 21:40:36 -0400, Peng Zang wrote: > > On Thursday 24 July 2008 06:32:36 pm Jon Harrop wrote: > > > There are some complications here. Firstly, NaN has multiple > > > representations that may or may not be physically equal but is always > > > structurally unequal to itself. > > > > Yeah, that always seemed broken to me. If two things are physically > > equal (they occupy the same memory space) it doesn't make sense for them > > to be structurally unequal (contain different content). Personally, one > > of the first things I did is redefined (=) to fix this. > > Humm... http://citeseer.ist.psu.edu/goldberg91what.html is certainly > a good read... > > My 0.02€, > C. I second that. Particularly relevant is how there is not one unique NaN but a family of them. OCaml has three treatments for them: 1) (=) treats all NaNs as different (even if they may in fact, at the bit level, be the same) 2) (compare) treats all NaNs as the same (even if they may in fact, at the bit level be different) 3) (==) two NaNs are equal iff they share the same memory (2) tends to come up the most often as you typically don't care what kind of NaN it is, as long as it is one. Peng -BEGIN PGP SIGNATURE- Version: GnuPG v2.0.7 (GNU/Linux) iD8DBQFIicFAfIRcEFL/JewRAiFkAJ46YFay72ONIDD7qk75/izt9ne8gACgvMzu EYZkQb7fYAlEU+KXnixp7to= =5iAB -END PGP SIGNATURE- ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] equality operators in OCaml
On Thu, 24 Jul 2008 21:40:36 -0400, Peng Zang wrote: > > On Thursday 24 July 2008 06:32:36 pm Jon Harrop wrote: > > There are some complications here. Firstly, NaN has multiple > > representations that may or may not be physically equal but is always > > structurally unequal to itself. > > Yeah, that always seemed broken to me. If two things are physically equal > (they occupy the same memory space) it doesn't make sense for them to be > structurally unequal (contain different content). Personally, one of the > first things I did is redefined (=) to fix this. Humm... http://citeseer.ist.psu.edu/goldberg91what.html is certainly a good read... My 0.02€, C. ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] equality operators in OCaml
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On Thursday 24 July 2008 06:32:36 pm Jon Harrop wrote: > From: "Raj Bandyopadhyay" <[EMAIL PROTECTED]> > > > Hi OCaml folk > > > > I apologize if I've been asking too many questions on this list recently, > > but I'm working on a heavy OCaml application and need help sometimes. > > > > I am having a disagreement with a colleague about how the equality > > operators in OCaml work and am trying to resolve it conclusively. > > > > 1) I understand that the '==' operator is basically a pointer comparison > > i.e. a==b true iff a and b are the same entity. Is this true? > > There are some complications here. Firstly, NaN has multiple > representations that may or may not be physically equal but is always > structurally unequal to itself. Yeah, that always seemed broken to me. If two things are physically equal (they occupy the same memory space) it doesn't make sense for them to be structurally unequal (contain different content). Personally, one of the first things I did is redefined (=) to fix this. Peng -BEGIN PGP SIGNATURE- Version: GnuPG v2.0.7 (GNU/Linux) iD8DBQFIiS8YfIRcEFL/JewRAnXqAKCAhHWGTHe6z58jRplmMmnNFgljUACfUhaJ SQSWrxxpPpHqiJcFBcNlVsM= =af4P -END PGP SIGNATURE- ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] equality operators in OCaml
From: "Raj Bandyopadhyay" <[EMAIL PROTECTED]> Hi OCaml folk I apologize if I've been asking too many questions on this list recently, but I'm working on a heavy OCaml application and need help sometimes. I am having a disagreement with a colleague about how the equality operators in OCaml work and am trying to resolve it conclusively. 1) I understand that the '==' operator is basically a pointer comparison i.e. a==b true iff a and b are the same entity. Is this true? There are some complications here. Firstly, NaN has multiple representations that may or may not be physically equal but is always structurally unequal to itself. Secondly, OCaml's implicit allocation can make "same entity" unclear, constant subexpressions (e.g. Some 3) may or may not be hoisted. The only place I ever use == and != is when optimizing code in such a way that unexpected results from those functions result in slow but still correct code. 2) Where can I find the code implementing the '==' operator in the OCaml code? Can you just compile it and look at the assembler? There are probably many different outputs depending upon the statically resolved types. 3) In case I do want to check object identity and can use the == operator, would it be faster to use '==', '=' or a match statement? I believe the == operator would be fastest. More often that not, you want < and > so you end up augmenting your OCaml data structures with unique IDs anyway. Note that physical equality does not even exist in SML and the designers of SML stand by that design decision. Perhaps it is best to avoid == in OCaml altogether. Cheers, Jon. ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] equality operators in OCaml
On Thu, Jul 24, 2008 at 02:27:51PM -0500, Raj Bandyopadhyay wrote: > Hi OCaml folk > > I apologize if I've been asking too many questions on this list > recently, but I'm working on a heavy OCaml application and need help > sometimes. > > I am having a disagreement with a colleague about how the equality > operators in OCaml work and am trying to resolve it conclusively. > > 1) I understand that the '==' operator is basically a pointer comparison > i.e. a==b true iff a and b are the same entity. Is this true? yes. only valid for object that are in blocks though (int, char are not). for object not in block, you end up with a simple = > 2) Where can I find the code implementing the '==' operator in the OCaml > code? byterun/compare.c : caml_equal which call compare_val with total=0 (which means to not compare inside blocks) > 3) In case I do want to check object identity and can use the == > operator, would it be faster to use '==', '=' or a match statement? i believe == is the fastest one. but that's just based on how the thing works compare to the others compare, not on empirical benchs. -- Vincent ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
