Joseph Ferano
d6fc15474b
The author: "I think I prefer length over len, because then I'll use len as the variable name". One arm in check.ml, one row in the table beside it, and every (len x) in lib, test, examples, vendor, spike, docs, web, emacs, plan.org and NEXT.md rewritten. Shadowing and builtin/ had already taken most of the sting out: a (defn len ...) was legal and won in its own file, and builtin/len reached past it. What was left is that len was still a builtin — the defn earned a warning, and a wrapper had to say builtin/ at every inner call. Now there is nothing under the short name: len is an ordinary identifier in every position, which is what (let [len (length xs)] ...) wants. length takes over as shadowing's worked example rather than the feature losing one. shadow-builtin.flan, builtin-qualified.flan, pkgs/shadowed and the builtin/ rows in test_flan move to it and go on testing shadowing. A call to a len nothing defines is answered where an unknown function is, after every table and after the shadowing guard, so a program with its own len never reaches it. The sentence is said rather than guessed at — len and length are three edits apart and the did-you-mean's net is one — and the call is written back out through spell_arg, as-slice's spelling lifted out of it and now shared, so what is printed compiles. sand.flan:33 still calls the old name and is the author's to change; until it does, test_acceptance and test_session abort there. Both were run green against a copy with that one line changed. FIX.org says so.
103 lines
3.7 KiB
Plaintext
103 lines
3.7 KiB
Plaintext
;;;; loop and recur.
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;;;;
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;;;; What is worth asserting here rather than in a unit test is the code that
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;;;; comes out, and there are four things:
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;;;;
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;;;; 1. A loop answers with the value of its body — the accumulator comes back
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;;;; without a mutable slot and without a sentinel flag.
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;;;; 2. recur rebinds every name *at once*. A swap is the test that fails if
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;;;; the writes were interleaved with the reads.
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;;;; 3. recur is a jump, not a call. A loop that goes round ten million times
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;;;; would overflow the stack if it were a call, and this one returns.
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;;;; 4. A loop whose body never falls off the end (every path recurs or
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;;;; returns) still terminates, which is the Never-bodied shape.
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(defn gcd [a i32 b i32] i32
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(loop [x a y b]
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(if (= y 0)
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x
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(recur y (% x y)))))
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;; The body is Never: neither arm produces a value, so there is no result slot
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;; and no break — nothing falls off the end of this loop.
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(defn first-over [n i32] i32
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(loop [i 0]
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(if (> (* i i) n)
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(return i)
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(recur (+ i 1)))))
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;; Named so the match below has a return type to read None out of.
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(defn step [i i32] (Option i32)
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(if (= i 4) None (Some i)))
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(defn main [] i32
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;; The value of the body, with no mutable accumulator anywhere.
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(print (loop [i 0 acc 0]
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(if (= i 5)
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acc
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(recur (+ i 1) (+ acc i)))))
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(println "") ; 0+1+2+3+4 = 10
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;; Simultaneous rebinding. Interleaved writes would give 1 1.
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(let [p (loop [a 1 b 2 n 0]
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(if (= n 3)
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a
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(recur b a (+ n 1))))]
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(print p) (println "")) ; three swaps: 2
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(print (gcd 1071 462)) (println "") ; 21
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(print (first-over 50)) (println "") ; 8
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;; A jump and not a call: ten million frames is not a stack this has.
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(print (loop [i 0]
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(if (= i 10000000) i (recur (+ i 1)))))
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(println "") ; 10000000
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;; recur in the tail of a let, and of a when inside a do — both are tails,
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;; and both are how a loop actually gets written.
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(print (loop [i 0 acc 1]
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(let [next (* acc 2)]
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(if (= i 6) acc (recur (+ i 1) next)))))
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(println "") ; 2^6 = 64
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;; A Unit-bodied loop: it is run for its effect and answers with nothing.
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(let [n 0]
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(loop [i 0]
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(when (< i 3)
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(print i)
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(recur (+ i 1))))
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(println "") ; 012
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(print n) (println "")) ; 0
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;; A while nested inside a loop keeps its own break: the loop is a barrier
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;; only to a jump that would *leave* it.
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(print (loop [i 0 acc 0]
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(if (= i 4)
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acc
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(let [j 0 hit 0]
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(while (< j 10)
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(set j (+ j 1))
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(when (= j 3) (set hit 1) (break)))
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(recur (+ i 1) (+ acc hit))))))
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(println "") ; 4
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;; A move-only accumulator, carried round by recur and answered with. This
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;; is the shape the form exists for: no mutable local, no sentinel flag, and
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;; the Vec is the loop's value. recur writes every name on the way round, so
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;; the "moves a value bound outside the loop" rule is not about acc.
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(let [v (loop [acc (vec-new i32) i 0]
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(if (= i 3)
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acc
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(do (push acc i) (recur acc (+ i 1)))))]
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(dotimes [i (length v)] (print (at v i)))
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(println "") ; 012
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(free v))
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;; A match arm is a tail too.
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(print (loop [i 0 acc 0]
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(match (step i)
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(Some v) (recur (+ i 1) (+ acc v))
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None acc)))
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(println "") ; 0+1+2+3 = 6
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0)
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