🚀 go-pugleaf

wg@opal.cs.tu-berlin.de (Wolfgang Grieskamp) writes:

>peterson-john@cs.yale.edu (John C. Peterson) writes:

>  >Regarding typing in Common Lisp & ML, it is incorrect to say that
>  >Common Lisp lacks types; type declarations in CL are actually far more
>  >general than those in ML.  What Common Lisp lacks is any sort of well
>  >defined type inference, a major deficiency.

>As i remember from a short afair with LISP some years ago, type 
>declarations in CL are always monomorphic: no parametric 
>polymorphy, no type variables. 

Oh sorry, someone inspired me meanwhile to remember how it works in CS: 
its actually much more general then even, say, dependent types. Its based 
roughly on the general program transformation scheme (excuse LISP style
errors):

	(def f '((T1 x1) ... (Tn xn)) E)
		=>
	(def f '(x1 ... xn) '(if (and (T1 x1) ... (Tn xn)) E '(halt)))

... where Ti's are user defined predicates (checking types) or primitives 
for the predefined types.

Now, the compiler can use abstract interpretation or other techniques to
analysis preconditions Ti and actual arguments to applications of f (if
he knows its an application) and generate type errors.  This is a bit hard 
sometimes, but will often work for predefined types like bool, int, etc. -
at least with the whole program under analysis.

Hope i understood the "typing" discipline in Common LISP correct know (the
LISP experts brought up typing and LISP should correct me otherwise).

--
Wolfgang Grieskamp 
wg@opal.cs.tu-berlin.de tub!tubopal!wg wg%opal@DB0TUI11.BITNET