[racket] shared and immutable structures

I am with Matthew. Immutable structs -- other than historically 'blessed' ones -- give you inductive reasoning. If you allow cycles anyway, that's out the window. 



On Feb 1, 2011, at 3:47 PM, Matthew Flatt wrote:

> At Tue, 1 Feb 2011 12:38:44 -0800, David Herman wrote:
>> Just thinking out loud, but is there any way to use the unsafe operations to 
>> make the shared library work for immutable data? If you can prove it's 
>> actually safe, then there'd be no harm to users. (After all, as you say, 
>> that's what it's doing with pairs.)
> 
> The difference is that pairs are defined a priori as allowing the
> possibility of cycles, so you know that possibility exists when working
> with pairs.
> 
> If my module gives yours a structure type, however, I may not want to
> automatically give yours the capability to create cyclic instances of
> the structure type. In that sense, creating a cycle is not always safe.
> 
> Furthermore, implementing the cycle by creation plus mutation would
> mean introducing mutation for a supposedly immutable datatype, which
> isn't necessarily safe if the intermediate state is visible. Attempting
> to hide the intermediate state creates all sorts of new problems for
> features like properties and guards.
> 
> Along those lines, granting the capability to create cyclic instances
> seems to be just about tantamount to granting mutability, so `shared'
> might as well require mutability.
> 
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