Subject: Re: Question: Lisp's power points From: Erik Naggum <email@example.com> Date: Sat, 13 Oct 2001 16:43:39 GMT Newsgroups: comp.lang.lisp Message-ID: <firstname.lastname@example.org> * Rolf Wester | My question was meant seriously, my point was not to gripe about anything. Then you deserve a better answer. | I'm not very familiar with CL but I'm fascinated by it's capabilities as | far as I recognized them. As I mostly programmed in Fortran/C++/Java | it's not so obvious for me to see in what respect Lisps level of | abstraction is much higher compared to other languages. This is actually a serious problem for Common Lisp, and you are far from alone in experiencing its effects. For instance, take special variables. People who do now know how they work, tend to dismiss them as some old Lisp concept that should not have survived. Then they write code that uses some global variables and they _set_ their values, they get yelled at for using global variables whose values are hard to track, and they think that global state is a bad thing. However, the bad thing about global variables is that they do _not_ have the infrstructure offered by special binding in Common Lisp. A function may legitimately desire to change the global state for a carefully controlled duration, but in order to do so, you have to guarantee that the old state is restored. This is truly hard if the language does not support it. Exceptions and other non-local control transfers intervene. Storing the variable in some local variable is not always trivial -- you need to replicate an object with the same type as the global variable, and the language does not give you that obvious operation, either -- and then you need to ensure that the stored value is put back into the global variable before you return, which can be foiled by the kind of unconscious programmers that are likely to mess with our code. So C++ people tend to think that special variables are useless, but in fact, they solve a very serious problem that C++ programmers do not generally try to solve because it is too hard and generally too painful in their language. Sticking to the convenient, they use semi-global state variables and are nervous. The abstraction offered by special variables is thus hard to grasp for the programmer who does not even perform the concrete operations the abstraction automates and hides. Even Common Lisp programmers are generally unaware of the concrete mechanisms underlying the abstraction. The impact on threads, for instance, is not quite obvious: One variable may be so global that it should affect all threads, while another may have a thread-local global value. This means that implementationally, a thread context switch needs to re-bind special variables if they share a Lisp image, and involve IPC and shared memory between threads if the variable is thread-global. This is so hairy stuff that even good programmers would shy away from thinking about using such things if they had to do it manually. To a Common Lisp programmer, it is a natural extension to special variables, even though conceptually much more advanced. Other forms of context switches may also be desirable, so it makes sense to expose the control mechanism for rebinding special variables, but this could now be done separately from the bindings because the infrastructure for bindings is already there. Thus the abstraction offered by special binding in Common Lisp spans an enormous amount of otherwise manual work, and they are not only "global variables", which is how some think of them, it is also a mechanism that can be used for variables that are _not_ global, but require the support of binding and unbinding of variables that can be shared only among a few functions that know about them. Therefore, there is a local declaration to request special binding of an otherwise lexical variable in addition to the global declaration to request it of all bindings of a symbols, and the global declaration applies to the symbol regardless of its context, because specialness of a symbol must be pervasive to work at all. This means that the mechanism behind the special binding is very different from the global variable in many other languages, which may be shadowed by a lexical binding (except they do not call it that). | P.S.: My mother language is not English so misunderstandings may arise | because I can't express myself in English as precisely as I could do in | German. It is probably OK to include the German if you are uncertain about an expression -- even if people here write in English, the language skills of the community is pretty amazing, and you could get tips and hints that would not get anywhere else. For instance, "Muttersprache" and "mother tongue" and "langue maternelle" have the common Latin etymology "lingua materna", but still it does not really work all that well to translate directly between them in any direction... /// -- The United Nations before and after the leadership of Kofi Annan are two very different organizations. The "before" United Nations did not deserve much credit and certainly not a Nobel peace prize. The "after" United Nations equally certainly does. I applaud the Nobel committee's choice.