Add -iterate, -unfold

README.md

@@ -72,6 +72,14 @@ Include this in your emacs settings to get syntax highlighting:
 * [-max](#-max-list) `(list)`
 * [-max-by](#-max-by-comparator-list) `(comparator list)`
 
+### Unfolding
+
+
+Operations dual to reductions, building lists from seed value rather than consuming a list to produce a single value.
+
+* [-iterate](#-iterate-fun-init-n) `(fun init n)`
+* [-unfold](#-unfold-fun-seed) `(fun seed)`
+
 ### Predicates
 
 * [-any?](#-any-pred-list) `(pred list)`
@@ -564,6 +572,45 @@ comparing them.
 ```
 
 
+## Unfolding
+
+
+Operations dual to reductions, building lists from seed value rather than consuming a list to produce a single value.
+
+#### -iterate `(fun init n)`
+
+Return a list of iterated applications of `fun` to `init`.
+
+This means a list of form:
+  '(init (fun init) (fun (fun init)) ...)
+
+`n` is the length of the returned list.
+
+```cl
+(-iterate '1+ 1 10) ;; => '(1 2 3 4 5 6 7 8 9 10)
+(-iterate (lambda (x) (+ x x)) 2 5) ;; => '(2 4 8 16 32)
+(--iterate (* it it) 2 5) ;; => '(2 4 16 256 65536)
+```
+
+#### -unfold `(fun seed)`
+
+Build a list from `seed` using `fun`.
+
+This is "dual" operation to `-reduce-r`: while -reduce-r
+consumes a list to produce a single value, `-unfold` takes a
+seed value and builds a (potentially infinite!) list.
+
+`fun` should return `nil` to stop the generating process, or a
+cons (`a` . `b`), where `a` will be prepended to the result and `b` is
+the new seed.
+
+```cl
+(-unfold (lambda (x) (unless (= x 0) (cons x (1- x)))) 10) ;; => '(10 9 8 7 6 5 4 3 2 1)
+(--unfold (when it (cons it (cdr it))) '(1 2 3 4)) ;; => '((1 2 3 4) (2 3 4) (3 4) (4))
+(--unfold (when it (cons it (butlast it))) '(1 2 3 4)) ;; => '((1 2 3 4) (1 2 3) (1 2) (1))
+```
+
+
 ## Predicates
 
 #### -any? `(pred list)`

dash.el

@@ -1089,6 +1089,45 @@ The items for the comparator form are exposed as \"it\" and \"other\"."
   (declare (debug (sexp form)))
   `(-min-by (lambda (it other) ,form) ,list))
 
+(defun -iterate (fun init n)
+  "Return a list of iterated applications of FUN to INIT.
+
+This means a list of form:
+  '(init (fun init) (fun (fun init)) ...)
+
+N is the length of the returned list."
+  (if (= n 0) nil
+    (let ((r (list init)))
+      (--dotimes (1- n)
+        (push (funcall fun (car r)) r))
+      (nreverse r))))
+
+(defmacro --iterate (form init n)
+  "Anaphoric version of `-iterate'."
+  (declare (debug (sexp form form)))
+  `(-iterate (lambda (it) ,form) ,init ,n))
+
+(defun -unfold (fun seed)
+  "Build a list from SEED using FUN.
+
+This is \"dual\" operation to `-reduce-r': while -reduce-r
+consumes a list to produce a single value, `-unfold' takes a
+seed value and builds a (potentially infinite!) list.
+
+FUN should return `nil' to stop the generating process, or a
+cons (A . B), where A will be prepended to the result and B is
+the new seed."
+  (let ((last (funcall fun seed)) r)
+    (while last
+      (push (car last) r)
+      (setq last (funcall fun (cdr last))))
+    (nreverse r)))
+
+(defmacro --unfold (form seed)
+  "Anaphoric version of `-unfold'."
+  (declare (debug (sexp form)))
+  `(-unfold (lambda (it) ,form) ,seed))
+
 (defun -cons-pair? (con)
   "Return non-nil if CON is true cons pair.
 That is (A . B) where B is not a list."
@@ -1367,6 +1406,10 @@ structure such as plist or alist."
                              "--max-by"
                              "-min-by"
                              "--min-by"
+                             "-iterate"
+                             "--iterate"
+                             "-unfold"
+                             "--unfold"
                              "-cons-pair?"
                              "-cons-to-list"
                              "-value-to-list"

dev/examples.el

@@ -199,6 +199,19 @@
     (--max-by (> (car it) (car other)) '((1 2 3) (2) (3 2))) => '(3 2)
     (--max-by (> (length it) (length other)) '((1 2 3) (2) (3 2))) => '(1 2 3)))
 
+(def-example-group "Unfolding"
+  "Operations dual to reductions, building lists from seed value rather than consuming a list to produce a single value."
+
+  (defexamples -iterate
+    (-iterate '1+ 1 10) => '(1 2 3 4 5 6 7 8 9 10)
+    (-iterate (lambda (x) (+ x x)) 2 5) => '(2 4 8 16 32)
+    (--iterate (* it it) 2 5) => '(2 4 16 256 65536))
+
+  (defexamples -unfold
+    (-unfold (lambda (x) (unless (= x 0) (cons x (1- x)))) 10) => '(10 9 8 7 6 5 4 3 2 1)
+    (--unfold (when it (cons it (cdr it))) '(1 2 3 4)) => '((1 2 3 4) (2 3 4) (3 4) (4))
+    (--unfold (when it (cons it (butlast it))) '(1 2 3 4)) => '((1 2 3 4) (1 2 3) (1 2) (1))))
+
 (def-example-group "Predicates" nil
   (defexamples -any?
     (-any? 'even? '(1 2 3)) => t