Thank you Pascal for your comments, very informative and enlightening. Alexandre Sent from my iPhone
> On 24 Nov 2018, at 14:10, Pascal Costanza <p...@p-cos.net> wrote: > > Hi, > >> On 24 Nov 2018, at 15:14, Alexandre Rademaker <aradema...@gmail.com> wrote: >> >> >> In the LoL book we learn the flexibility of the Common Lisp macro system. >> Reading now >> >> http://blog.racket-lang.org/2011/04/writing-syntax-case-macros.html >> >> It seems that the Racket macros do not necessarily impose constraints to the >> programmers. But I never really used it. Does anyone here have experiences >> with both approaches, sufficient to give insights about the good and bad of >> each one? > > > syntax-case is, strictly speaking, strictly more expressive than Common Lisp > macros. If it is worth it to switch to syntax-case because of this, is a > different question though, because you can make an argument that the increase > in expressiveness doesn’t solve an essential problem. > > syntax-case solves a particular case of macro hygiene issues. (syntax-rules > solves it too, but removes a lot of other possibilities to express things, > whereas syntax-case doesn’t have such restrictions.) > > The problem can be illustrated with a simple example: > > (defun foo () > (let ((x 42)) > (macrolet ((bar () ‘x)) > (let ((x 4711)) > (bar))))) > > It can be argued, especially if you’re a very strong believer that everything > should be lexically scoped without exception, that (foo) should return 42, > because the bar macro apparently refers to the variable named x bound to 42. > Alas, (foo) actually returns 4711, and Common Lisp provides no way to solve > this /in the language/. (That last part, “in the language”, is very important > when speaking of expressiveness. There are very good pragmatic solutions to > this issue which, however, only work by convention and are not based on > explicit language mechanisms. My favorite solution is to just use different > names for the different bindings. ;) > > With syntax-case and its default notations, which look very different from > the Common Lisp macro system, (foo) would by default return 42, but there are > mechanisms in syntax-case to also return 4711 if you explicitly ask for it, > without renaming any of the variables. > > There is a very interesting paper by Daniel Friedman called “Object-Oriented > Style” - see https://www.cs.indiana.edu/~dfried/ - where he implements an > object system completely in syntax-case (that is, without the need for any > particular runtime support), and where a variant of this issue pops up. I > found it very enlightening to try to reimplement this with Common Lisp’s > macro system, because you can actually get very far with it. (There are some > tricks using &env-based environment objects, macroexpand with explicit > passing of such environment objects, and symbol-macrolet, but you will > ultimately arrive at a stumbling block where this doesn’t work anymore.) > > This paper motivated me to actually embed hygienic macros in Common Lisp. > This is described in > http://www.jucs.org/jucs_16_2/embedding_hygiene_compatible_macros > > However, this is not a solution “in the language” because this only works > reliably if you write your whole programs in that system. > > I’m still not convinced that hygienic macro systems like syntax-case are > worth the complexity. Again, just don’t reuse variable names, and you should > be fine. (For global variables, it’s easy to use the Common Lisp package > system to solve hygiene issues.) > > Just my 0.02€. > > > Pascal > > -- > Pascal Costanza > >
