On 20 March 2014 17:04, Eliot Miranda <eliot.mira...@gmail.com> wrote:
> > > > On Thu, Mar 20, 2014 at 2:03 AM, Christophe Demarey < > christophe.dema...@inria.fr> wrote: > >> Hi Eliot, >> >> Le 19 mars 2014 à 16:25, Eliot Miranda a écrit : >> >> Hi Christophe, >> >> On Mar 19, 2014, at 1:45 AM, Christophe Demarey < >> christophe.dema...@inria.fr> wrote: >> >> >> Le 18 mars 2014 à 19:50, Eliot Miranda a écrit : >> >> On Tue, Mar 18, 2014 at 10:10 AM, Christophe Demarey < >> christophe.dema...@inria.fr> wrote: >> >>> Why arguments should be read only? >>> They are just temporary variables with an initial value. >>> >> >> Read the blue book. It was a decision of the language designers to >> forbid assignment to arguments to allow debugging. The assignment to block >> arguments is a side-effect of the old BlockContext implementation of blocks >> where block arguments were mapped onto temporary variables of the home >> context. It is an anachronism and should be forbidden also. >> >> >> Thank you for the explanation. >> I'm just curious why it is so difficult to implement a debugger if >> arguments are assignable? >> If you need to restart the execution of a method, and so you need to get >> the initial value of the argument, I understand you cannot find the value >> anymore in the method context but it is available in the caller context, no? >> As I never implemented a debugger, I cannot figure out the difficulties. >> >> >> the args are no longer available; they get moved from the caller context >> to the callee. If you think about stack frames then what happens is that >> the slots containing the outgoing arguments are used as the slots for the >> incoming >> arguments. So if arguments are assigned to they are indeed lost. >> >> >> ok, I understand. Thank you for the explanation. >> >> >> But look at how many methods in the system (or in any system). The >> proportion of methods/functions/procedures that could be written to assign >> their arguments is very small so the Smalltalk trade off is a good one. >> >> >> I agree we don't really loose something. You can always assign arguments >> to temporary variables and update temporary variables. My point was just: >> if possible, why don't do it? >> > > It's that tricky trade-off between value and cost. Is it worth it? Does > doing it provide as much value as doing what it displaces (opportunity > cost). IMO it simply doesn't provide enough value, and there are different > priorities in the compiler. But if you disagree implement it your self and > then it'll be done. > > self := super agreed. > -- > best, > Eliot > -- Best regards, Igor Stasenko.
