> what Common Lisp offers, especially SBCL: optional static typing
Common Lisp offers optionally unsound static typing.
(SBCL doesn't change that, that's why it inserts runtime checks.)
Common Lisp offers optionally unsound static typing.
(SBCL doesn't change that, that's why it inserts runtime checks.)
SBCL does offer optional static typing as it goes beyond Common Lisp and can perform static type checking across compilation units.
If you have a lisp file like:
(defun foo (x) (declare (fixnum x)) (* x 2))
(defun bar () (foo 3.5))
; file: foo.lisp
; in: DEFUN BAR
; (FOO 3.5)
;
; note: deleting unreachable code
;
; caught WARNING:
; Asserted type FIXNUM conflicts with derived type
; (VALUES (SINGLE-FLOAT 3.5 3.5) &OPTIONAL).
; See also:
; The SBCL Manual, Node "Handling of Types"If some part of your program extracts values from a heterogeneous list and calls foo on one of them, SBCL will not be able to statically guarantee that it is a FIXNUM at runtime. It can't reject the program either, that would require seriously subsetting the language.
In that case, everything extracted from a list is T and you'd have to get that warning you mentioned above.
Such warnings would very likely be false-positive, unless Common Lisp programs are buggy as hell most of the time—assuming that would be the real FUD, no? Therefore, the non-FUD conclusion is to expect a high rate of false-positive warnings in typical list-using programs.
TBH, I suspect those false-positive warnings you mentioned aren't bona fide warnings, but some kind optimizer stream-of-consciousness log stream.
(defun sum (lst)
(let ((acc 0))
(dolist (x lst)
(setq acc (+ x acc)))
acc))
(sum '("a" "b" "c"))
Common Lisp has a (CONS α β) type for cons cells with member types α and β, but its LIST type is defined as (OR (CONS T T) NULL), basically more of a binary tree type, where T is the Common Lisp "any" union of all types.
A useable (LIST α) type would be something like μβ.(OR (CONS α β) NULL), but that's not expressible within standard Common Lisp. I expect it to very hard to infer too, especially with CONS cells being mutable.