[racket] shared and immutable structures

Because I taught -- and heavily exploited -- SHARED before we covered
mutation, I would have found it very helpful to keep the two concepts
distinct.  Making cycles depend on mutation seemed to undo some of the
fabulous pedagogic benefit that SHARED provides (that people in other
languages can only dream of).

Shriram

On Tue, Feb 1, 2011 at 3:58 PM, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
> 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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