add defcached for memoization

for #84

ciel.asd

@@ -142,6 +142,8 @@
                :serapeum
                :shlex
 
+               :function-cache  ;; memoization
+
                ;; tests
                :fiveam
 

docs/language-extensions.md

@@ -183,6 +183,8 @@ And there is more. All the available macros are:
 
 ## Functions
 
+### Partial application
+
 We import Serapeum's `partial` and Alexandria's `rcurry`. They allow
 partial application of functions.
 
@@ -194,6 +196,25 @@ You can check [Serapeum's function helpers](https://github.com/ruricolist/serape
 the non-imported ones are only a `serapeum` package prefix away in CIEL if you need
 them. Other noticeable functions: `distinct`, `once`, `throttle`, `trampoline`, `do-nothing`…
 
+### Memoization
+
+We import `defcached` from
+[function-cache](https://github.com/AccelerationNet/function-cache), a
+library that provides extensible caching/memoization.
+
+For example:
+
+```lisp
+(defcached (foo :timeout 10) (arg)
+  (sleep 3)
+  arg)
+```
+
+Run `(foo 1)`, it sleeps 3 seconds and returns 1. Run `(foo 1)` again
+and it returns 1 immediately. Its result (for this argument) is cached
+for 10 seconds.
+
+
 
 ## Bind, more destructuring in `let` (metabang-bind)
 

src/ciel.lisp

@@ -290,6 +290,11 @@ We currently only try this with serapeum. See *deps/serapeum/sequences-hashtable
 (export '(apropos-regex
           apropos-regex-list))
 
+;; function-cache: memoization
+(cl-reexport:reexport-from :function-cache
+                           :include '(:defcached))  ;; not in binary??
+
+
 ;;;
 ;;; Conveniently add type declarations.
 ;;; Straight from Serapeum, only it is -> thus it conflicts with our arrow-macro.