Joseph Ferano
f7009fcd34
The checker tests pin the reason rather than the failure: an array pattern over a slice has to fail *because a slice's length is a runtime value*, not because something went wrong. The four map-destructuring keys Clojure has and this does not are each named individually, because "unexpected form" leaves the author guessing which of the four they wrote is the missing one. The acceptance program exists for the case none of the above can see. A pattern is desugared away entirely, so there is nothing in the typed IR to inspect; the only way to tell that the value was bound once is to destructure something with a side effect and print how often it ran. Four names, two calls. A desugaring that re-evaluated the initialiser per name prints 4, and every other line in the program stays green through the mistake.
97 lines
3.6 KiB
Plaintext
97 lines
3.6 KiB
Plaintext
;;;; Destructuring in a let: Clojure's binding forms over Flan's shapes.
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;;;;
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;;;; A struct stands in for Clojure's map, so {:keys [x y]} and {inner :field}
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;;;; read fields off one; a fixed array stands in for its sequence, so [a b]
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;;;; and [a b & rest] read elements out of one. None of it is a new form: it
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;;;; all desugars in parse.ml into the Let, (.field x), at and slice that were
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;;;; already there, which is why this program is the test that it works — the
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;;;; typed IR has nothing in it a pattern could be hiding in.
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;;;;
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;;;; The case that matters most here is `calls`. A pattern binds several names
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;;;; from one value, and that value is bound to a temporary *first*, so a
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;;;; pattern over a call calls it once. Delete the temporary and every name
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;;;; re-evaluates the initialiser: this program prints the call count, so that
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;;;; mistake changes the output instead of hiding in it.
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(defstruct Point [x i32 y i32])
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(defstruct Line [a Point b Point])
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(defvar calls i32)
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(defn make-point [] Point
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(set calls (+ calls 1))
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(Point {:x 3 :y 4}))
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(defn show2 [label string a i32 b i32]
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(print-str label)
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(print-str " ")
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(print-i64 (i64 a))
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(print-str " ")
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(print-i64 (i64 b))
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(newline))
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(defn main [] i32
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;; :keys, the common case: one name per field, spelled as the field is.
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(let [{:keys [x y]} (Point {:x 1 :y 2})]
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(show2 "keys" x y))
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;; The pair form, which is what renames and what nests — a :keys entry is a
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;; field name and never a pattern.
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(let [{a :x b :y} (Point {:x 10 :y 20})]
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(show2 "pairs" a b))
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(let [l (Line {:a (Point {:x 5 :y 6}) :b (Point {:x 7 :y 8})})]
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(let [{{:keys [x y]} :b} l]
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(show2 "nested" x y))
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;; A pattern may shadow the very name it destructures, because the value is
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;; read into a temporary before any of the names are bound.
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(let [{l :a} l]
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(show2 "shadow" (.x l) (.y l))))
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;; A later binding sees an earlier pattern's names, as in any let.
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(let [{:keys [x]} (Point {:x 100 :y 0})
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doubled (* x 2)]
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(show2 "sequential" x doubled))
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;; A fixed array names every element. The count is checked against the type,
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;; so [a b] over a [3 i32] is a compile error and not a silent prefix.
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(let [xs [11 22 33]
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[a b c] xs]
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(print-str "array ")
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(print-i64 (i64 a)) (print-str " ")
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(print-i64 (i64 b)) (print-str " ")
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(print-i64 (i64 c)) (newline))
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;; & rest is the tail as a slice, which is an ordinary (slice xs n (len xs))
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;; over a local — nothing new, and nothing that outlives the array.
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(let [xs [1 2 3 4 5]
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[head & tail] xs]
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(print-str "rest ")
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(print-i64 (i64 head)) (print-str " ")
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(print-i64 (i64 (len tail))) (print-str " ")
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(print-i64 (i64 (at tail 0))) (print-str " ")
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(print-i64 (i64 (at tail 3))) (newline))
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;; The tail may be empty: naming every element and then asking for the rest
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;; is a zero-length slice, not an error.
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(let [xs [9 8]
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[p q & rest] xs]
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(print-str "empty-tail ")
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(print-i64 (i64 (+ p q))) (print-str " ")
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(print-i64 (i64 (len rest))) (newline))
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;; Patterns nest through each other: a struct inside an array.
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(let [ps [(Point {:x 1 :y 2}) (Point {:x 3 :y 4})]
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[{:keys [x]} {y :y}] ps]
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(show2 "nested-in-array" x y))
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;; Evaluate-once. Two patterns, two calls, four names — one call per pattern.
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;; Without the temporary each of the four names would call it again: 4, not 2.
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(let [{:keys [x y]} (make-point)
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{a :x b :y} (make-point)]
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(print-str "calls ")
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(print-i64 (i64 calls)) (print-str " ")
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(print-i64 (i64 (+ x (+ y (+ a b)))))
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(newline))
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0)
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