Add -tails and -inits

README.md

@@ -208,6 +208,8 @@ Operations pretending lists are sets.
 * [-intersection](#-intersection-list-list2) `(list list2)`
 * [-powerset](#-powerset-list) `(list)`
 * [-permutations](#-permutations-list) `(list)`
+* [-inits](#-inits-list) `(list)`
+* [-tails](#-tails-list) `(list)`
 * [-distinct](#-distinct-list) `(list)`
 
 ### Other list operations
@@ -1481,6 +1483,26 @@ Return the permutations of `list`.
 (-permutations '(a b c)) ;; => '((a b c) (a c b) (b a c) (b c a) (c a b) (c b a))
 ```
 
+#### -inits `(list)`
+
+Return all non-empty prefixes of `list`.
+
+```el
+(-inits '(1 2 3 4)) ;; => '((1) (1 2) (1 2 3) (1 2 3 4))
+(-inits nil) ;; => nil
+(-inits '(1)) ;; => '((1))
+```
+
+#### -tails `(list)`
+
+Return all non-empty suffixes of `list`
+
+```el
+(-tails '(1 2 3 4)) ;; => '((1 2 3 4) (2 3 4) (3 4) (4))
+(-tails nil) ;; => nil
+(-tails '(1)) ;; => '((1))
+```
+
 #### -distinct `(list)`
 
 Return a new list with all duplicates removed.

dash.el

@@ -2075,6 +2075,17 @@ or with `-compare-fn' if that's non-nil."
                              (-permutations (remove x list))))
                    list))))
 
+(defun -inits (list)
+  "Return all non-empty prefixes of LIST."
+  (nreverse (-map 'reverse (-tails (nreverse list)))))
+
+(defun -tails (list)
+  "Return all non-empty suffixes of LIST"
+  (-reduce-r-from
+   (lambda (it acc)
+     (cons (cons it (car acc)) acc))
+   nil list))
+
 (defun -contains? (list element)
   "Return non-nil if LIST contains ELEMENT.
 

dash.texi

@@ -2209,6 +2209,46 @@ Return the permutations of @var{list}.
 @end example
 @end defun
 
+@anchor{-inits}
+@defun -inits (list)
+Return all non-empty prefixes of @var{list}.
+
+@example
+@group
+(-inits '(1 2 3 4))
+    @result{} '((1) (1 2) (1 2 3) (1 2 3 4))
+@end group
+@group
+(-inits nil)
+    @result{} nil
+@end group
+@group
+(-inits '(1))
+    @result{} '((1))
+@end group
+@end example
+@end defun
+
+@anchor{-tails}
+@defun -tails (list)
+Return all non-empty suffixes of @var{list}
+
+@example
+@group
+(-tails '(1 2 3 4))
+    @result{} '((1 2 3 4) (2 3 4) (3 4) (4))
+@end group
+@group
+(-tails nil)
+    @result{} nil
+@end group
+@group
+(-tails '(1))
+    @result{} '((1))
+@end group
+@end example
+@end defun
+
 @anchor{-distinct}
 @defun -distinct (list)
 Return a new list with all duplicates removed.

dev/examples.el

@@ -607,6 +607,16 @@ new list."
     (-permutations '(1 2)) => '((1 2) (2 1))
     (-permutations '(a b c)) => '((a b c) (a c b) (b a c) (b c a) (c a b) (c b a)))
 
+  (defexamples -inits
+    (-inits '(1 2 3 4)) => '((1) (1 2) (1 2 3) (1 2 3 4))
+    (-inits nil) => nil
+    (-inits '(1)) => '((1)))
+
+  (defexamples -tails
+    (-tails '(1 2 3 4)) => '((1 2 3 4) (2 3 4) (3 4) (4))
+    (-tails nil) => nil
+    (-tails '(1)) => '((1)))
+
   (defexamples -distinct
     (-distinct '()) => '()
     (-distinct '(1 2 2 4)) => '(1 2 4)))