Joseph Ferano
a4c6b996ff
The trio the author decided on 2026-09-20 is now all built: def is CL's defparameter — its initialiser runs on every daemon re-run, unguarded, so an edited initialiser repaints the same storage on C-c C-c plus re-run — defonce (Clojure's name for CL's defvar, per the author) initialises once behind the .init~once. flag, and defconst stays the image. One parse arm reads both forms; the difference is Ast.reinit, carried to Tast.global's grerun. Emit.startup_plan gives a def no guard flag, and Check.check_global lifts every def initialiser — zero and literal included — into global/<n>, so the host's startup reaches it through the function cell and a re-evaluated def swaps it (Session's def_inits; Emit.redefinition declares the cell for a non-sibling target). The old defvar spelling is refused with the rename and both compiling spellings, and every program, test, doc and editor list is swept — except sand.flan, the author's live WIP, whose seven defvar lines are flagged in FIX.org and keep its three dependent tests red on this branch.
119 lines
4.4 KiB
Plaintext
119 lines
4.4 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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(defonce 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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;; The shorthand: a lone .field with no name before it binds a local of the
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;; field's own name, which is what :keys does and in the spelling the rest of
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;; the language uses for a field. It mixes with the pair form in one brace,
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;; because the two are read one item at a time and a dot in head position is
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;; the only thing that tells them apart.
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(let [{.x .y} (Point {.x 30 .y 40})]
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(show2 "shorthand" x y))
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(let [{.x b .y} (Point {.x 50 .y 60})]
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(show2 "shorthand-mixed" x 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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