Joseph Ferano
57fe91f303
FIX.org, NEXT.md, DISCUSS.org, docs/DISCUSS.md and the session handoff at the root are one TODO.org now: 293 entries under seven subsystem headings, each carrying an org keyword that says where it stands. A DONE entry is a few lines saying what was decided and what that rules out; the reasoning that would not compress — the embedding spike and the four reports the hand-written x86 backend was built from — moved into docs/BUILT.md instead, and its entries point there in one line. Every entry was checked against the tree before it got a keyword, and the prose was wrong in both directions. Things the deleted files called open were built: the first-evaluation stall, main being redefinable, macro parameter lists, the type-limit constants, the array constructors, the byte fills, inc/dec, the discard's fontification, the Emacs buffers, rt_die's _exit, the backtrace surface, and the acceptance failure that could print and still exit zero. Things they called done were not: the backend reports' no-plan buckets had gone stale in the other direction, the value-dependent defvar was superseded rather than built, and macro-expansion source locations are on an unmerged lane, so that entry is NEXT and names the branch. Every comment that cited one of the five by name now cites a heading that exists, in TODO.org or in docs/BUILT.md. The session reports under docs/handoffs/ keep naming the files they worked on, because rewriting them would falsify what those sessions did; each carries a note saying where the content went.
62 lines
2.6 KiB
Plaintext
62 lines
2.6 KiB
Plaintext
;;;; (slice-from-ptr p n) — TODO.org, "A pointer from C needs a length before
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;;;; it can be indexed". The motivating pointers come from C, but nothing about
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;;;; the form does: a (Ptr T) is a (Ptr T) whoever made it, so this case makes
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;;;; its own with (addr (at a 0)) and needs no library and no window.
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;;;;
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;;;; What is asserted, line by line:
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;;;;
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;;;; - the length is the one the caller stated, and (length s) answers it;
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;;;; - the elements read through are the same storage, not a copy — the last
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;;;; two lines write through the slice and read the array back, which is
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;;;; the whole ptr+len claim;
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;;;; - a shorter promise than the truth is legal and is what indexing then
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;;;; believes, because the caller's number is the only length there is;
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;;;; - the result is an ordinary [T]: it slices, it is passed to a function
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;;;; that takes a slice, and the prelude's algorithms work on it.
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;;;;
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;;;; What is NOT here, and is in test_flan.ml's refusal table instead: a
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;;;; negative literal length, a first argument that is not a pointer, and
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;;;; (free (slice-from-ptr ...)) — a slice owns nothing, so free refuses it by
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;;;; the rule it already had.
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(defonce a [5 i32])
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(defn total [s [i32]] i32
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(let [acc 0]
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(dotimes [i (length s)]
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(set acc (+ acc (at s i))))
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acc))
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(defn main [] ()
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(dotimes [i 5]
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(set (at a i) (* (+ i 1) 10)))
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;; The whole array, as the caller promises it: five elements behind the
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;; address of the first.
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(let [s (slice-from-ptr (addr (at a 0)) 5)]
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(println (length s)) ; 5
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(println (at s 0)) ; 10
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(println (at s 4)) ; 50
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(println (total s)) ; 150
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;; An ordinary [i32] from here on: slice it, and the sub-view still points
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;; into the same storage.
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(println (total (slice s 1 3)))) ; 50
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;; A promise shorter than the truth. Nothing complains — there is nothing to
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;; complain with — and the length the caller gave is the length indexing and
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;; the bounds check both use.
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(let [s (slice-from-ptr (addr (at a 1)) 2)]
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(println (length s)) ; 2
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(println (total s))) ; 50
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;; Zero is a length like any other. An empty slice is not a null pointer and
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;; is not an error.
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(println (length (slice-from-ptr (addr (at a 0)) 0))) ; 0
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;; It is a view, not a copy: a write through the slice is a write to the
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;; array, and this is what would fail if the form ever grew a memcpy.
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(let [s (slice-from-ptr (addr (at a 0)) 5)]
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(set (at s 2) 7)
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(println (at a 2)) ; 7
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(println (total s)))) ; 127
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