Re: redo-safe-variables and redo-safe-parameters

[Noah Lavine]

On Wed, Mar 27, 2013 at 11:04 AM, Stefan Israelsson Tampe <address@hidden> wrote:

Hi Noha,

On Wed, Mar 27, 2013 at 3:29 PM, Noah Lavine <address@hidden> wrote:
> Hello,
>
>
> On Wed, Mar 27, 2013 at 9:22 AM, Stefan Israelsson Tampe
> <address@hidden> wrote:

> I don't understand the difference. If I use ~, I get redo-safe behavior, and
> if I use !, I get regular behavior (value shared between dynamic states).
> Can I use ~ and ! on the same variable at different places in the code? If
> yes, doesn't it have to switch behavior?

using set! means that you basically destroys the redo safe property.
There is no sound concept where you mix them. If you want to mix them
use ~ and add correct function guards to describe the semantics.

...

You want to allow a user to let one variable behave as with set! and one as with
set~. It is not broken, the useres sees ~ on one of the varibles and !
on the other.

Yes, I agree. What I'm saying is, there should be two different ways to declare the variables, and once a variable is declared, you should not need to look elsewhere in the code to see whether it acts like a regular or redo-safe variable. If that is what you specified, I apologize, but I thought that it wasn't.

In particular, I think that having an MIT-Scheme-style fluid-let will do the right thing here. I would be interested in talking about its interaction with closure variables, but I think that it's the right thing here with regard to continuations and mutable state.

 

BTW. srfi's shouuld be careful about specifying dynamic state in order
to achieve thread safe concpets, Scheme48 is threadsafe with their
fluid-let, guile would not be.

Yes, that's an interesting point.

Noah