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. -- best, Eliot
