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+;;; list.el --- Functions for working with lists -*- lexical-binding: t -*-
+
+;; Author: William Carroll <wpcarro@gmail.com>
+;; Version: 0.0.1
+;; Package-Requires: ((emacs "24"))
+
+;;; Commentary:
+;; Since I prefer having the `list-' namespace, I wrote this module to wrap many
+;; of the functions that are defined in the the global namespace in Elisp.  I
+;; sometimes forget the names of these functions, so it's nice for them to be
+;; organized like this.
+;;
+;; Motivation:
+;; Here are some examples of function names where I prefer more modern
+;; alternatives:
+;; - `car': Return the first element (i.e. "head") of a linked list
+;; - `cdr': Return the tail of a linked list
+
+;; As are most APIs for standard libraries that I write, this is influenced by
+;; Elixir's standard library.
+;;
+;; Similar libraries:
+;; - dash.el: Excellent and widely adopted library for working with lists.
+;; - list-utils.el: Utility library that covers things that dash.el may not
+;;   cover.
+
+;;; Code:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'maybe)
+(require 'set)
+(require 'set)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defun list-new ()
+  "Return a new, empty list."
+  '())
+
+(defun list-concat (&rest lists)
+  "Joins `LISTS' into on list."
+  (apply #'append lists))
+
+(defun list-duplicate (n x)
+  "Duplicates the given element, X, N times in a list."
+  (list-map (lambda (_) x) (number-sequence 1 n)))
+
+(defun list-join (joint xs)
+  "Join a list of strings, XS, with JOINT."
+  (if (list-empty? xs)
+      ""
+    (list-reduce (list-first xs)
+                 (lambda (x acc)
+                   (format "%s%s%s" acc joint x))
+                 (list-tail xs))))
+
+(defun list-length (xs)
+  "Return the number of elements in `XS'."
+  (length xs))
+
+(defun list-get (i xs)
+  "Return the value in `XS' at `I', or nil."
+  (nth i xs))
+
+(defun list-first (xs &optional default)
+  "Alias for `list-head' for `XS'."
+  (if (list-empty? xs)
+      default
+    (car xs)))
+
+(defun list-last (xs &optional default)
+  "Returns the last element in XS or DEFAULT if empty."
+  (if (list-empty? xs)
+      default
+    (nth (- (length xs) 1) xs)))
+
+(defun list-tail (xs)
+  "Return the tail of `XS'."
+  (cdr xs))
+
+(defun list-reverse (xs)
+  "Reverses `XS'."
+  (reverse xs))
+
+(defun list-cons (x xs)
+  "Add `X' to the head of `XS'."
+  (cons x xs))
+
+(defun list-delete (x xs)
+  "Deletes the given element, X, from XS.
+Returns a new list without X. If X occurs more than once, only the first
+  occurrence is removed."
+  (let ((deleted? nil))
+    (list-reject (lambda (y)
+                   (if deleted? nil
+                     (when (equal x y)
+                       (setq deleted? t) t)))
+                 xs)))
+
+(defun list-filter (p xs)
+  "Return a subset of XS where predicate P returned t."
+  (list--assert-instance xs)
+  (seq-filter p xs))
+
+(defun list-map (f xs)
+  "Call `F' on each element of `XS'."
+  (list--assert-instance xs)
+  (seq-map f xs))
+
+(defun list-reduce (acc f xs)
+  "Return over `XS' calling `F' on an element in `XS'and `ACC'."
+  (list--assert-instance xs)
+  (seq-reduce (lambda (acc x) (funcall f x acc)) xs acc))
+
+(defun list-map-indexed (f xs)
+  "Call `F' on each element of `XS' along with its index."
+  (list-reverse
+   (cdr
+    (list-reduce '(0 . nil)
+                 (lambda (x acc)
+                   (let ((i (car acc))
+                         (result (cdr acc)))
+                     `(,(+ 1 i) . ,(cons (funcall f x i) result))))
+                 xs))))
+
+(defun list-reject (p xs)
+  "Return a subset of XS where predicate of P return nil."
+  (list-filter (lambda (x) (not (funcall p x))) xs))
+
+(defun list-find (p xs)
+  "Return the first x in XS that passes P or nil."
+  (list--assert-instance xs)
+  (seq-find p xs))
+
+(defun list-dedupe-adjacent (xs)
+  "Return XS without adjacent duplicates."
+  (list-reverse
+   (list-reduce (list (list-first xs))
+                (lambda (x acc)
+                  (if (equal x (list-first acc))
+                      acc
+                    (list-cons x acc)))
+                xs)))
+
+(defun list-chunk (n xs)
+  "Chunk XS into lists of size N."
+  (if (> n (length xs))
+      (list xs)
+    (let* ((xs (list-reduce '(:curr () :result ())
+                            (lambda (x acc)
+                              (let ((curr (plist-get acc :curr))
+                                    (result (plist-get acc :result)))
+                                (if (= (- n 1) (length curr))
+                                    `(:curr () :result ,(list-cons (list-reverse (list-cons x curr)) result))
+                                  `(:curr ,(list-cons x curr) :result
+                                          ,result)))) xs))
+           (curr (plist-get xs :curr))
+           (result (plist-get xs :result)))
+      (list-reverse (if curr (list-cons curr result) result)))))
+
+(defun list-wrap (xs)
+  "Wraps XS in a list if it is not a list already."
+  (if (list-instance? xs)
+      xs
+    (list xs)))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Predicates
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defun list-instance? (xs)
+  "Return t if `XS' is a list.
+Be leery of using this with things like alists.  Many data structures in Elisp
+  are implemented using linked lists."
+  (listp xs))
+
+(defun list-empty? (xs)
+  "Return t if XS are empty."
+  (= 0 (list-length xs)))
+
+(defun list-all? (p xs)
+  "Return t if all `XS' pass the predicate, `P'."
+  (if (list-empty? xs)
+      t
+    (and (maybe-some? (funcall p (car xs)))
+         (list-all? p (cdr xs)))))
+
+(defun list-any? (p xs)
+  "Return t if any `XS' pass the predicate, `P'."
+  (if (list-empty? xs)
+      nil
+    (or (maybe-some? (funcall p (car xs)))
+        (list-any? p (cdr xs)))))
+
+(defun list-contains? (x xs)
+  "Return t if X is in XS using `equal'."
+  (list--assert-instance xs)
+  (maybe-some? (seq-contains-p xs x)))
+
+(defun list-xs-distinct-by? (f xs)
+  "Return t if all elements in XS are distinct after applying F to each."
+  (= (length xs)
+     (set-count (set-from-list (list-map f xs)))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Helpers
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defun list--assert-instance (xs)
+  (unless (list-instance? xs)
+    (error (format "Assertion failed: argument is not a list: %s" xs))))
+
+(provide 'list)
+;;; list.el ends here