pmonks · December 7, 2025 02:27
diff --git a/once.clj b/once.clj
 ;
 ; Copyright © 2025 Peter Monks
 ;
 ; This Source Code Form is subject to the terms of the Mozilla Public
 ; License, v. 2.0. If a copy of the MPL was not distributed with this
 ; file, You can obtain one at https://mozilla.org/MPL/2.0/.
 ;
 ; SPDX-License-Identifier: MPL-2.0
 ;

 ; Requires this dependency (i.e. in `deps.edn` or equivalent):
 ;
 ;      org.clojure/core.cache {:mvn/version "1.1.234"}  ; or whatever the latest version is

 (ns com.github.pmonks.once
  "Macro that provides soft 'once' semantics for the forms passed to it."
  (:require [clojure.core.cache.wrapped :as cache]))

 ; We use a soft reference cache here, so that the JVM has free rein to garbage
 ; collect some or all of the cache if there's memory pressure.  A different
 ; core.cache implementation could be chosen if different eviction semantics are
 ; preferred - the once macro is not sensitive to the cache implementation.
 (def cache (cache/soft-cache-factory {}))

 (defmacro once
  "On first use evaluates `forms`, caches the result, then returns it.  Returns
  that cached result on subsequent uses (though depending on the core.cache
  cache implementation the cached result may have been evicted, in which case
  the forms will be re-evaluated)."
  [& forms]
 ;  (let [cache-key (random-uuid)]   ; Note: Clojure 1.11+ only
  (let [cache-key (java.util.UUID/randomUUID)]
    `(cache/lookup-or-miss cache ~cache-key (fn [_#] ~@forms))))
	;
	; Copyright © 2025 Peter Monks
	;
	; This Source Code Form is subject to the terms of the Mozilla Public
	; License, v. 2.0. If a copy of the MPL was not distributed with this
	; file, You can obtain one at https://mozilla.org/MPL/2.0/.
	;
	; SPDX-License-Identifier: MPL-2.0
	;

	; Requires this dependency (i.e. in `deps.edn` or equivalent):
	;
	; org.clojure/core.cache {:mvn/version "1.1.234"} ; or whatever the latest version is

	(ns com.github.pmonks.once
	"Macro that provides soft 'once' semantics for the forms passed to it."
	(:require [clojure.core.cache.wrapped :as cache]))

	; We use a soft reference cache here, so that the JVM has free rein to garbage
	; collect some or all of the cache if there's memory pressure. A different
	; core.cache implementation could be chosen if different eviction semantics are
	; preferred - the once macro is not sensitive to the cache implementation.
	(def cache (cache/soft-cache-factory {}))

	(defmacro once
	"On first use evaluates `forms`, caches the result, then returns it. Returns
	that cached result on subsequent uses (though depending on the core.cache
	cache implementation the cached result may have been evicted, in which case
	the forms will be re-evaluated)."
	[& forms]
	; (let [cache-key (random-uuid)] ; Note: Clojure 1.11+ only
	(let [cache-key (java.util.UUID/randomUUID)]
	`(cache/lookup-or-miss cache ~cache-key (fn [_#] ~@forms))))
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