Joseph Ferano
c3319e6268
Three things the review found, and the refusal it was right about. The returned-Vec refusal is gone. It called a leak a dangle: the storage a returned Vec owns outlives the expression, so the view reads what it says it reads, and what is lost is the owner. (len (mk)) and (at (mk) 0) lose the same owner and compile, spec-memory.md already says an overwritten global Vec leaks its first block, and under a region there is nothing to leak at all. "We're purposely doing manual memory management for the static side, so whatever." The array refusal stays exactly as it is — that one is a view into a frame that is gone and answers bytes the frame has since reused. Wrong answers are the compiler's business and leaks are the program's, and both docs now draw that line, because the two forms look alike. The -1 sentinel was reachable from user syntax: (slice v 0 -1) answered the whole Vec on both backends while (slice a 0 -1) was refused as a negative bound. The refusal now runs on the bounds the reader wrote, before the implicit hi is built — the only order that works, since the sentinel is itself a -1 and a check on the finished pair would refuse (slice v). The backwards-pair check moved into the branch where both ends are written. The bounds seam is closed toward index_expr, and the tiebreaker is not which half is older. indexed and vec_at both take their index through it, so (at a c) over a u32 compiled where (slice a c) did not: the fork was between slice and at as much as between two targets. A bound is a subscript. Also an x86 row for vec.flan, so the new arity is pinned on both backends in CI rather than by hand, and the comment columns the sweep shifted left in slurp, into, format and algorithms.
104 lines
4.5 KiB
Plaintext
104 lines
4.5 KiB
Plaintext
;;;; slurp and barf — NEXT.md decisions 2 and 5.
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;;;;
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;;;; slurp reads a whole file and answers a (Vec u8). It allocates, which is
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;;;; why it waited for Vec, and it follows spec-memory.md's rule to the letter:
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;;;; no allocating operation returns an error, so there is no Result here and
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;;;; no out-parameter anywhere.
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;;;;
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;;;; Failure signals a condition under a restart, which is the pattern
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;;;; StorageExhausted set this session. Two restarts, and they are the pair
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;;;; Common Lisp establishes for a file-error:
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;;;;
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;;;; retry the file may be there now
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;;;; use-value [p string] try this other path instead
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;;;;
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;;;; use-value is a *typed* restart — the second thing this session bought —
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;;;; and it is the first one the compiler itself emits. Its parameter is the
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;;;; path slot the attempt reads, so the clause body is empty: the invoker's
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;;;; argument lands in the slot, the clause falls through, and the loop
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;;;; re-attempts against the new path.
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;; Handlers cannot see the locals of the function that established them, so the
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;; observations are globals — the same shape exhausted.flan uses.
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(defonce seen i64)
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(defonce last-reason i32)
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(defonce last-op i32)
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(defonce last-path string)
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(defn main [] i32
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;; ── The happy path ────────────────────────────────────────────────
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(let [v (slurp "programs/assets/a.txt")]
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(println (len v)) ; 13
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(print (string (slice v))) ; hello from a
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(free v))
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;; Byte-exact, the same as an embed: nothing here decodes anything.
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(let [v (slurp "programs/assets/raw.bin")]
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(println (len v)) ; 4
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(println (at v 0)) ; 0
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(println (at v 2)) ; 255
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(free v))
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;; ── use-value: a missing file, answered with another path ─────────
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;; The textbook case. The handler does not know what slurp was going to do
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;; with the bytes and does not have to: it names a file that is there and
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;; the read resumes as if that had been asked for all along.
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(handler-bind
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[(FileError [c]
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(set seen (+ seen 1))
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(set last-reason (.reason c))
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(set last-op (.op c))
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(set last-path (.path c))
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(invoke-restart 'use-value "programs/assets/b.bin"))]
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(let [v (slurp "programs/assets/does-not-exist")]
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(println (len v)) ; 3
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(println (string (slice v))) ; BBB
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(free v)))
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(println seen) ; 1
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(println (= last-reason file-missing)) ; true
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(println (= last-op file-op-read)) ; true
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;; The condition carries the path that actually failed, not the one that
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;; eventually worked — the handler is told what it is answering about.
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(println last-path) ; programs/assets/does-not-exist
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;; ── barf, and reading back what it wrote ──────────────────────────
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(barf "slurp-out.txt" (bytes-view "round trip\n"))
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(let [v (slurp "slurp-out.txt")]
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(println (len v)) ; 11
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(print (string (slice v))) ; round trip
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(free v))
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;; barf's own failure signals the same condition with op = write. A directory
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;; that does not exist is the reachable case on every platform.
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(set seen 0)
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(handler-bind
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[(FileError [c]
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(set seen (+ seen 1))
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(set last-op (.op c))
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(invoke-restart 'use-value "slurp-out.txt"))]
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(barf "no-such-dir/x.txt" (bytes-view "second\n")))
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(println seen) ; 1
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(println (= last-op file-op-write)) ; true
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(let [v (slurp "slurp-out.txt")]
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(print (string (slice v))) ; second
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(free v))
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;; ── retry: the file was not there, so the handler makes it ────────
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;; The other restart, and the one use-value cannot stand in for: here the
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;; path is right and the world is wrong. The handler fixes the world and
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;; re-attempts the *same* request, which is exactly what retry means for a
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;; failed allocation too.
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(set seen 0)
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(handler-bind
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[(FileError [c]
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(set seen (+ seen 1))
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(set last-reason (.reason c))
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(barf "slurp-made.txt" (bytes-view "made by the handler\n"))
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(invoke-restart 'retry))]
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(let [v (slurp "slurp-made.txt")]
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(print (string (slice v))) ; made by the handler
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(free v)))
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(println seen) ; 1
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(println (= last-reason file-missing)) ; true
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0)
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