;;; 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 that I cannot tolerate:
;; - `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 heavily
;; influenced by Elixir's standard library.
;;
;; Elixir's List library:
;; - ++/2
;; - --/2
;; - hd/1
;; - tl/1
;; - in/2
;; - length/1
;;
;; Similar libraries:
;; - dash.el: Functional library that mimmicks Clojure. It is consumed herein.
;; - list-utils.el: Utility library that covers things that dash.el may not
;; cover.
;; stream.el: Elisp implementation of streams, "implemented as delayed
;; evaluation of cons cells."
;; TODO: Consider naming this file linked-list.el.
;; TODO: Support module-like macro that auto-namespaces functions.
;; TODO: Consider wrapping most data structures like linked-lists,
;; associative-lists, etc in a `cl-defstruct', so that the dispatching by type
;; can be nominal instead of duck-typing. I'm not sure if this is a good idea
;; or not. If I do this, I should provide isomorphisms to map between idiomatic
;; ways of working with Elisp data structures and my wrapped variants.
;; TODO: Are function aliases/synonyms even a good idea? Or do they just
;; bloat the API unnecessarily?
;;; Code:
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;; Dependencies
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(require 'maybe)
(require 'set)
(require 'set)
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;; Library
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(defun list-new ()
"Return a new, empty list."
'())
(defun list-concat (&rest lists)
"Joins `LISTS' into on list."
(apply #'-concat 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))
;; TODO: Support dedupe.
;; TODO: Should we call this unique? Or distinct?
;; TODO: Add tests.
(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)))))
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;; Predicates
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(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)))))
(defun list-wrap (xs)
"Wraps XS in a list if it is not a list already."
(if (list-instance? xs)
xs
(list xs)))
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;; Helpers
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(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
