Joseph Ferano
9a820d86cd
The script is in tools/ rather than thrown away, because two lanes are
writing Flan in the old spelling right now and their files need the same
pass at merge.
It works on forms, not on text: a keyword becomes a dot only where it sits
in a field-label position inside a brace, so an enum member in value
position, a map key inside an EDN string and a type-position {K V} are all
left alone. :keys keeps its colon -- it names no field.
109 lines
3.9 KiB
Plaintext
109 lines
3.9 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 label)
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(print " ")
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(print a)
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(print " ")
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(print b)
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(println ""))
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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 "array ")
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(print a) (print " ")
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(print b) (print " ")
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(print c) (println ""))
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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 "rest ")
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(print head) (print " ")
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(print (len tail)) (print " ")
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(print (at tail 0)) (print " ")
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(print (at tail 3)) (println ""))
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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 "empty-tail ")
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(print (+ p q)) (print " ")
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(print (len rest)) (println ""))
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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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;; A tail of something wider than a machine word. The corpus slices arrays of
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;; i32, u8 and f32 and nothing else, so this is the one place the desugared
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;; (slice xs n (len xs)) has to get a struct's stride right rather than a
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;; scalar's.
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(let [ps [(Point {.x 1 .y 2}) (Point {.x 3 .y 4}) (Point {.x 5 .y 6})]
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[first & others] ps]
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(print "struct-tail ")
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(print (.x first)) (print " ")
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(print (len others)) (print " ")
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(print (.y (at others 0))) (print " ")
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(print (.x (at others 1))) (println ""))
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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 "calls ")
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(print calls) (print " ")
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(print (+ x (+ y (+ a b))))
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(println ""))
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0)
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