Add -prodfn

README.md

@@ -199,6 +199,7 @@ These combinators require Emacs 24 for its lexical scope. So they are offered in
 * [-orfn](#-orfn-rest-preds) `(&rest preds)`
 * [-andfn](#-andfn-rest-preds) `(&rest preds)`
 * [-iteratefn](#-iteratefn-fn-n) `(fn n)`
+* [-prodfn](#-prodfn-rest-fns) `(&rest fns)`
 
 ## Anaphoric functions
 
@@ -1687,6 +1688,27 @@ This function satisfies the following law:
 (funcall (-iteratefn 'cdr 3) '(1 2 3 4 5)) ;; => '(4 5)
 ```
 
+#### -prodfn `(&rest fns)`
+
+Take a list of n functions and return a function that takes a
+list of length n, applying i-th function to i-th element of the
+input list.  Returns a list of length n.
+
+In types (for n=2): ((a -> b), (c -> d)) -> (a, c) -> (b, d)
+
+This function satisfies the following laws:
+
+  (-compose (-prodfn f g ...) (-prodfn f' g' ...)) = (-prodfn (-compose f f') (-compose g g') ...)
+  (-prodfn f g ...) = (-juxt (-compose f (-partial 'nth 0)) (-compose g (-partial 'nth 1)) ...)
+  (-compose (-prodfn f g ...) (-juxt f' g' ...)) = (-juxt (-compose f f') (-compose g g') ...)
+  (-compose (-partial 'nth n) (-prod f1 f2 ...)) = (-compose fn (-partial 'nth n))
+
+```cl
+(funcall (-prodfn '1+ '1- 'int-to-string) '(1 2 3)) ;; => '(2 1 "3")
+(-map (-prodfn '1+ '1-) '((1 2) (3 4) (5 6) (7 8))) ;; => '((2 1) (4 3) (6 5) (8 7))
+(apply '+ (funcall (-prodfn 'length 'string-to-int) '((1 2 3) "15"))) ;; => 18
+```
+
 
 ## Contribute
 

dash-functional.el

@@ -135,6 +135,21 @@ This function satisfies the following law:
   (funcall (-iteratefn fn n) init) = (-last-item (-iterate fn init (1+ n)))."
   (lambda (x) (--dotimes n (setq x (funcall fn x))) x))
 
+(defun -prodfn (&rest fns)
+  "Take a list of n functions and return a function that takes a
+list of length n, applying i-th function to i-th element of the
+input list.  Returns a list of length n.
+
+In types (for n=2): ((a -> b), (c -> d)) -> (a, c) -> (b, d)
+
+This function satisfies the following laws:
+
+  (-compose (-prodfn f g ...) (-prodfn f' g' ...)) = (-prodfn (-compose f f') (-compose g g') ...)
+  (-prodfn f g ...) = (-juxt (-compose f (-partial 'nth 0)) (-compose g (-partial 'nth 1)) ...)
+  (-compose (-prodfn f g ...) (-juxt f' g' ...)) = (-juxt (-compose f f') (-compose g g') ...)
+  (-compose (-partial 'nth n) (-prod f1 f2 ...)) = (-compose fn (-partial 'nth n))"
+  (lambda (x) (-zip-with 'funcall fns x)))
+
 (provide 'dash-functional)
 
 ;;; dash-functional.el ends here

dev/examples.el

@@ -725,4 +725,26 @@
                     (-last-item (-iterate fn init (1+ 3))))
              (equal (funcall (-iteratefn fn 5) init)
                     (-last-item (-iterate fn init (1+ 5)))))))
+
+    (defexamples -prodfn
+      (funcall (-prodfn '1+ '1- 'int-to-string) '(1 2 3)) => '(2 1 "3")
+      (-map (-prodfn '1+ '1-) '((1 2) (3 4) (5 6) (7 8))) => '((2 1) (4 3) (6 5) (8 7))
+      (apply '+ (funcall (-prodfn 'length 'string-to-int) '((1 2 3) "15"))) => 18
+      (let ((f '1+)
+            (g '1-)
+            (ff 'string-to-int)
+            (gg 'length)
+            (input '(1 2))
+            (input2 "foo")
+            (input3 '("10" '(1 2 3))))
+        (equal (funcall (-prodfn f g) input)
+               (funcall (-juxt (-compose f (-partial 'nth 0)) (-compose g (-partial 'nth 1))) input))
+        (equal (funcall (-compose (-prodfn f g) (-juxt ff gg)) input2)
+               (funcall (-juxt (-compose f ff) (-compose g gg)) input2))
+        (equal (funcall (-compose (-partial 'nth 0) (-prod f g)) input)
+               (funcall (-compose f (-partial 'nth 0)) input))
+        (equal (funcall (-compose (-partial 'nth 1) (-prod f g)) input)
+               (funcall (-compose g (-partial 'nth 1)) input))
+        (equal (funcall (-compose (-prodfn f g) (-prodfn ff gg)) input3)
+               (funcall (-prodfn (-compose f ff) (-compose g gg)) input3))))
     ))