[Caml-list] comparing references

[Caml-list] comparing references

[Tahir H Butt]

I don't know for sure, so I decided to ask around. My question is this:
given two references, when one compares them, is it just the addressing
which is examined, or the actual content? For instance:

type symbol = Symbol of int
let a = ref (Symbol 1) in
let b = ref (Symbol 2) in
let c = ref (Symbol 1) in
....

upon inspection, a=c -> true, and a=b -> false, as desired. But, is this
comparison as simple as comparing two numbers? I ask because my sum
types can be big (I'm dealing with trees), so it would be good if when I
want to check if two trees are the same, all I need to do is look at the
references to them.

Thanks,

Tahir H Butt
Brown University
Cognitive and Linguistic Sciences
PhD Student
http://www.cog.brown.edu/~tahir

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Re: [Caml-list] comparing references

[Michal Moskal]

On Mon, Aug 11, 2003 at 02:54:47PM -0400, Tahir H Butt wrote:
> I don't know for sure, so I decided to ask around. My question is this:
> given two references, when one compares them, is it just the addressing
> which is examined, or the actual content? For instance:
> 
> type symbol = Symbol of int
> let a = ref (Symbol 1) in
> let b = ref (Symbol 2) in
> let c = ref (Symbol 1) in
> ....
> 
> upon inspection, a=c -> true, and a=b -> false, as desired. 

= does structural equality. As ref is implemented as record type, there
is nothing that would prevent = from traversing entire ref-ed data structure.

> But, is this
> comparison as simple as comparing two numbers? 

It can take more time. It is even possible it won't stop on cyclic
data structures.

> I ask because my sum
> types can be big (I'm dealing with trees), so it would be good if when I
> want to check if two trees are the same, all I need to do is look at the
> references to them.

Use == if you want to compare addresses.

-- 
: Michal Moskal :: http://www.kernel.pl/~malekith : GCS {C,UL}++++$ a? !tv
: When in doubt, use brute force. -- Ken Thompson : {E-,w}-- {b++,e}>+++ h

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Re: [Caml-list] comparing references

[Hal Daume III]

> > I ask because my sum
> > types can be big (I'm dealing with trees), so it would be good if when I
> > want to check if two trees are the same, all I need to do is look at the
> > references to them.
> 
> Use == if you want to compare addresses.

How much do you need to worry about, ex., the GC moving stuff around and
messing this up?

 - Hal


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Re: [Caml-list] comparing references

[Michal Moskal]

On Mon, Aug 11, 2003 at 12:57:46PM -0700, Hal Daume III wrote:
> > > I ask because my sum
> > > types can be big (I'm dealing with trees), so it would be good if when I
> > > want to check if two trees are the same, all I need to do is look at the
> > > references to them.
> > 
> > Use == if you want to compare addresses.
> 
> How much do you need to worry about, ex., the GC moving stuff around and
> messing this up?

You don't need to worry at all. GC moves all references to given object
at once. So comparison results are always the same.

-- 
: Michal Moskal :: http://www.kernel.pl/~malekith : GCS {C,UL}++++$ a? !tv
: When in doubt, use brute force. -- Ken Thompson : {E-,w}-- {b++,e}>+++ h

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