Elnatan Reisner wrote:
> On Sun, 2009-08-09 at 21:09 +0200, Alain Frisch wrote:
>> On 8/9/2009 8:56 PM, Elnatan Reisner wrote:
>>> My other issue is that the description of (==) for mutable structures
>>> doesn't specify that it is symmetric; reading the documentation
>>> literally only implies that e1 is a substructure of e2. Even just adding
>>> 'and vice versa' might clean this up:
>>> |e1 == e2| is true if and only if physical modification of |e1| also
>>> affects |e2 and vice versa|
>> It depends on what 'physical modification' and 'affect' mean. Clearly, 
>> the documentation means toplevel modifications of the values (i.e. 
>> modifying fields for record values, or elements for arrays or strings). 
>> If one includes deep modifications, then your extended criterion does 
>> not work either (think about two mutually recursive records).
> 
> You're right; thanks for pointing this out. But what does this mean for
> physical equality? What does it really mean? Does [e1 == e2] mean e1 and
> e2 are the same entity in memory---i.e., they are equal as C pointers?
> 
>> Note that (=) sometimes terminates for cylic values.
>>
>> # type t = A of t | B of t;;
>> type t = A of t | B of t
>> # (let rec x = A x in x) = (let rec x = B x in x);;
>> - : bool = false
> 
> Again, thanks for pointing this out. But can (=) ever evaluate to true
> on cyclic structures?

Yes:

let rec x = `A x;;
let o = object val x = x end;;
o = o;;

-> true


Martin

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