Add delay (#574)

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 [Squint](https://github.com/squint-cljs/squint): Light-weight ClojureScript dialect
 
+## v0.8.124 (2024-11-08)
+
+- Add `delay`
+
 ## v0.8.123 (2024-11-05)
 
 - Fix [#572](https://github.com/squint-cljs/squint/issues/572): prevent vite page reload

resources/squint/core.edn

@@ -1,5 +1,6 @@
 #{Atom
   Cons
+  Delay
   IIterable
   IIterable__iterator
   LazySeq

src/squint/compiler.cljc

@@ -95,7 +95,8 @@
                              'or macros/core-or
                              'and macros/core-and
                              'assert macros/core-assert
-                             'simple-benchmark macros/simple-benchmark}
+                             'simple-benchmark macros/simple-benchmark
+                             'delay macros/delay}
                             cc/common-macros))
 
 (def core-config {:vars (edn-resource "squint/core.edn")})

src/squint/core.js

@@ -2707,3 +2707,18 @@ export function update_vals(m, f) {
   }, m2, m);
   return m2;
 }
+
+export class Delay {
+  constructor(f) {
+    this.f = f;
+  }
+  _deref() {
+    if (this.realized) {
+      return this.v;
+    } else {
+      this.v = this.f();
+      this.realized = true;
+      return this.v;
+    }
+  }
+}

src/squint/internal/macros.cljc

@@ -14,7 +14,7 @@
                             bit-and-not bit-clear bit-flip bit-test
                             bit-shift-left bit-shift-right bit-shift-right-zero-fill
                             unsigned-bit-shift-right bit-set undefined?
-                            simple-benchmark])
+                            simple-benchmark delay])
   (:require [clojure.string :as str]
             [squint.compiler-common :as-alias ana]
             [clojure.core :as cc]
@@ -595,3 +595,11 @@
              elapsed# (- end# start#)]
          (~print-fn (str ~bs-str ", " ~expr-str ", "
                          ~iterations " runs, " elapsed# " msecs"))))))
+
+(core/defmacro delay
+  "Takes a body of expressions and yields a Delay object that will
+  invoke the body only the first time it is forced (with force or deref/@), and
+  will cache the result and return it on all subsequent force
+  calls."
+  [& body]
+  `(new cljs.core/Delay (fn [] ~@body) nil))

test/squint/compiler_test.cljs

@@ -2293,5 +2293,9 @@ new Foo();")
 (deftest issue-537-test
   (is (true? (jsv! "(not (= 1 2))"))))
 
+(deftest delay-test
+  (is (eq [0 2 1 2 1]
+          (jsv! "(def a (atom 0)) (def x (delay (do (swap! a inc) 2))) [@a @x @a @x @a]"))))
+
 (defn init []
   (t/run-tests 'squint.compiler-test 'squint.jsx-test 'squint.string-test 'squint.html-test))