62 lines
2.6 KiB
Plaintext
62 lines
2.6 KiB
Plaintext
;;;; (slice-from p n) — TODO.org, "A pointer from C needs a length before
|
|
;;;; it can be indexed". The motivating pointers come from C, but nothing about
|
|
;;;; the form does: a (Ptr T) is a (Ptr T) whoever made it, so this case makes
|
|
;;;; its own with (addr (at a 0)) and needs no library and no window.
|
|
;;;;
|
|
;;;; What is asserted, line by line:
|
|
;;;;
|
|
;;;; - the length is the one the caller stated, and (length s) answers it;
|
|
;;;; - the elements read through are the same storage, not a copy — the last
|
|
;;;; two lines write through the slice and read the array back, which is
|
|
;;;; the whole ptr+len claim;
|
|
;;;; - a shorter promise than the truth is legal and is what indexing then
|
|
;;;; believes, because the caller's number is the only length there is;
|
|
;;;; - the result is an ordinary [T]: it slices, it is passed to a function
|
|
;;;; that takes a slice, and the prelude's algorithms work on it.
|
|
;;;;
|
|
;;;; What is NOT here, and is in test_flan.ml's refusal table instead: a
|
|
;;;; negative literal length, a first argument that is not a pointer, and
|
|
;;;; (free (slice-from ...)) — a slice owns nothing, so free refuses it by
|
|
;;;; the rule it already had.
|
|
|
|
(defonce a [5 i32])
|
|
|
|
(defn total [s [i32]] i32
|
|
(let [acc 0]
|
|
(dotimes [i (length s)]
|
|
(set acc (+ acc (at s i))))
|
|
acc))
|
|
|
|
(defn main [] ()
|
|
(dotimes [i 5]
|
|
(set (at a i) (* (+ i 1) 10)))
|
|
|
|
;; The whole array, as the caller promises it: five elements behind the
|
|
;; address of the first.
|
|
(let [s (slice-from (addr (at a 0)) 5)]
|
|
(println (length s)) ; 5
|
|
(println (at s 0)) ; 10
|
|
(println (at s 4)) ; 50
|
|
(println (total s)) ; 150
|
|
;; An ordinary [i32] from here on: slice it, and the sub-view still points
|
|
;; into the same storage.
|
|
(println (total (slice s 1 3)))) ; 50
|
|
|
|
;; A promise shorter than the truth. Nothing complains — there is nothing to
|
|
;; complain with — and the length the caller gave is the length indexing and
|
|
;; the bounds check both use.
|
|
(let [s (slice-from (addr (at a 1)) 2)]
|
|
(println (length s)) ; 2
|
|
(println (total s))) ; 50
|
|
|
|
;; Zero is a length like any other. An empty slice is not a null pointer and
|
|
;; is not an error.
|
|
(println (length (slice-from (addr (at a 0)) 0))) ; 0
|
|
|
|
;; It is a view, not a copy: a write through the slice is a write to the
|
|
;; array, and this is what would fail if the form ever grew a memcpy.
|
|
(let [s (slice-from (addr (at a 0)) 5)]
|
|
(set (at s 2) 7)
|
|
(println (at a 2)) ; 7
|
|
(println (total s)))) ; 127
|