On Wed, 25 May 2011 07:34:22 +0200, Pascal Costanza said:
On 25 May 2011, at 04:51, Matthew D. Swank wrote:
I have implemented a small fexpr interpreter in Common Lisp based on Kernel http://web.cs.wpi.edu/~jshutt/kernel.html. Right now it's a Lisp 1, but I am considering trying to make it a more idiomatic extension of Common Lisp by making it a Lisp 2.
Part of the Lisp 2-ness of the Common Lisp evaluator is that the car of a form must name a function, macro, or special form. This name is either a symbol bound in the function name space, or a lambda expression. However, Kernel just requires the car evaluate to a combiner (this is what Kernel calls a generic operator). Obviously, in a Lisp 2, a symbol would evaluate to the value bound to it in the function name space. However, consider the following:
((returns-a-function) arg arg ...)
Would it be reasonable to allow this as a legal form as well?
I'm not arguing Common Lisp should work this way, but I seems to make sense in the context of a Kernel like evaluator.
_If_ (returns-a-function) indeed returns a function, then this could be ok. But what if it doesn't return a function? What if it is a macro that returns just a symbol? Do you want to risk that ((return-something) ...) has a different meaning than (funcall (return-something) ...)? This is potentially confusing and could lead to code that is hard to debug...
Maybe no harder than other Lisp-2's :-)
It seems like an interesting extension of the term "Lisp-2" -- as well as just specifying different namespaces, the proposal is to have a different evaluator for the car of the form. In fact, CL already has one, but it is very limited.
If that evaluator allows macro forms, then a macro that expands to a symbol should work as if the symbol was used directly. If you use such a macro with (funcall (return-something) ...), then you are using the regular evaluator for the macro form.