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? Christophe.
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