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+;;; al.el --- Interface for working with associative lists -*- lexical-binding: t -*-
+
+;; Author: William Carroll <wpcarro@gmail.com>
+;; Version: 0.0.1
+;; Package-Requires: ((emacs "25.1"))
+
+;;; Commentary:
+;; Firstly, a rant:
+;; In most cases, I find Elisp's APIs to be confusing.  There's a mixture of
+;; overloaded functions that leak the implementation details (TODO: provide an
+;; example of this.) of the abstract data type, which I find privileges those
+;; "insiders" who spend disproportionately large amounts of time in Elisp land,
+;; and other functions with little-to-no pattern about the order in which
+;; arguments should be applied.  In theory, however, most of these APIs could
+;; and should be much simpler.  This module represents a step in that direction.
+;;
+;; I'm modelling these APIs after Elixir's APIs.
+;;
+;; On my wishlist is to create protocols that will allow generic interfaces like
+;; Enum protocols, etc.  Would be nice to abstract over...
+;; - associative lists (i.e. alists)
+;; - property lists (i.e. plists)
+;; - hash tables
+;; ...with some dictionary or map-like interface.  This will probably end up
+;; being quite similar to the kv.el project but with differences at the API
+;; layer.
+;;
+;; Similar libraries:
+;; - map.el: Comes bundled with recent versions of Emacs.
+;; - asoc.el: Helpers for working with alists.  asoc.el is similar to alist.el
+;;   because it uses the "!" convention for signalling that a function mutates
+;;   the underlying data structure.
+;; - ht.el: Hash table library.
+;; - kv.el: Library for dealing with key-value collections.  Note that map.el
+;;   has a similar typeclass because it works with lists, hash-tables, or
+;;   arrays.
+;; - a.el: Clojure-inspired way of working with key-value data structures in
+;; Elisp.  Works with alists, hash-tables, and sometimes vectors.
+;;
+;; Some API design principles:
+;; - The "noun" (i.e. alist) of the "verb" (i.e. function) comes last to improve
+;; composability with the threading macro (i.e. `->>') and to improve consumers'
+;; intuition with the APIs.  Learn this once, know it always.
+;;
+;; - Every function avoids mutating the alist unless it ends with !.
+;;
+;; - CRUD operations will be named according to the following table:
+;;   - "create" *and* "set"
+;;   - "read"   *and* "get"
+;;   - "update"
+;;   - "delete" *and* "remove"
+;;
+;; For better or worse, all of this code expects alists in the form of:
+;; ((first-name . "William") (last-name . "Carroll"))
+;;
+;; Special thanks to github.com/alphapapa/emacs-package-dev-handbook for some of
+;; the idiomatic ways to update alists.
+;;
+;; TODO: Include a section that compares alist.el to a.el from
+;; github.com/plexus/a.el.
+
+;;; Code:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies:
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'dash)
+(require 'list)
+(require 'map)
+
+;; TODO: Support function aliases for:
+;; - create/set
+;; - read/get
+;; - update
+;; - delete/remove
+
+;; Support mutative variants of functions with an ! appendage to their name.
+
+;; Ensure that the same message about only updating the first occurrence of a
+;; key is consistent throughout documentation using string interpolation or some
+;; other mechanism.
+
+;; TODO: Consider wrapping all of this with `(cl-defstruct alist xs)'.
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; TODO: Support a variadic version of this to easily construct alists.
+(defun al-new ()
+  "Return a new, empty alist."
+  '())
+
+;; Create
+;; TODO: See if this mutates.
+(defun al-set (k v xs)
+  "Set K to V in XS."
+  (if (al-has-key? k xs)
+      (progn
+        ;; Note: this is intentional `alist-get' and not `al-get'.
+        (setf (alist-get k xs) v)
+        xs)
+    (list-cons `(,k . ,v) xs)))
+
+(defun al-set! (k v xs)
+  "Set K to V in XS mutatively.
+Note that this doesn't append to the alist in the way that most alists handle
+  writing.  If the k already exists in XS, it is overwritten."
+  (map-delete xs k)
+  (map-put! xs k v))
+
+;; Read
+(defun al-get (k xs &optional default)
+  "Return the value at K in XS; otherwise, return nil or DEFAULT (if set).
+Returns the first occurrence of K in XS since alists support multiple entries."
+  (if (not (al-has-key? k xs))
+      default
+    (cdr (assoc k xs))))
+
+(defun al-get-entry (k xs)
+  "Return the first key-value pair at K in XS."
+  (assoc k xs))
+
+;; Update
+;; TODO: Add warning about only the first occurrence being updated in the
+;; documentation.
+(defun al-update (k f xs)
+  "Apply F to the value stored at K in XS.
+If `K' is not in `XS', this function errors.  Use `al-upsert' if you're
+interested in inserting a value when a key doesn't already exist."
+  (if (not (al-has-key? k xs))
+      (error "Refusing to update: key does not exist in alist")
+    (al-set k (funcall f (al-get k xs)) xs)))
+
+(defun al-update! (k f xs)
+  "Call F on the entry at K in XS.
+Mutative variant of `al-update'."
+  (al-set! k (funcall f (al-get k xs))xs))
+
+;; TODO: Support this.
+(defun al-upsert (k v f xs)
+  "If K exists in `XS' call `F' on the value otherwise insert `V'."
+  (if (al-has-key? k xs)
+      (al-update k f xs)
+    (al-set k v xs)))
+
+;; Delete
+;; TODO: Make sure `delete' and `remove' behave as advertised in the Elisp docs.
+(defun al-delete (k xs)
+  "Deletes the entry of K from XS.
+This only removes the first occurrence of K, since alists support multiple
+  key-value entries.  See `al-delete-all' and `al-dedupe'."
+  (remove (assoc k xs) xs))
+
+(defun al-delete! (k xs)
+  "Delete the entry of K from XS.
+Mutative variant of `al-delete'."
+  (delete (assoc k xs) xs))
+
+;; Additions to the CRUD API
+;; TODO: Implement this function.
+(defun al-dedupe-keys (xs)
+  "Remove the entries in XS where the keys are `equal'.")
+
+(defun al-dedupe-entries (xs)
+  "Remove the entries in XS where the key-value pair are `equal'."
+  (delete-dups xs))
+
+(defun al-keys (xs)
+  "Return a list of the keys in XS."
+  (mapcar 'car xs))
+
+(defun al-values (xs)
+  "Return a list of the values in XS."
+  (mapcar 'cdr xs))
+
+(defun al-has-key? (k xs)
+  "Return t if XS has a key `equal' to K."
+  (not (eq nil (assoc k xs))))
+
+(defun al-has-value? (v xs)
+  "Return t if XS has a value of V."
+  (not (eq nil (rassoc v xs))))
+
+(defun al-count (xs)
+  "Return the number of entries in XS."
+  (length xs))
+
+;; TODO: Should I support `al-find-key' and `al-find-value' variants?
+(defun al-find (p xs)
+  "Find an element in XS.
+
+Apply a predicate fn, P, to each key and value in XS and return the key of the
+first element that returns t."
+  (let ((result (list-find (lambda (x) (funcall p (car x) (cdr x))) xs)))
+    (if result
+        (car result)
+      nil)))
+
+(defun al-map-keys (f xs)
+  "Call F on the values in XS, returning a new alist."
+  (list-map (lambda (x)
+              `(,(funcall f (car x)) . ,(cdr x)))
+            xs))
+
+(defun al-map-values (f xs)
+  "Call F on the values in XS, returning a new alist."
+  (list-map (lambda (x)
+              `(,(car x) . ,(funcall f (cdr x))))
+            xs))
+
+(defun al-reduce (acc f xs)
+  "Return a new alist by calling F on k v and ACC from XS.
+F should return a tuple.  See tuple.el for more information."
+  (->> (al-keys xs)
+       (list-reduce acc
+                    (lambda (k acc)
+                      (funcall f k (al-get k xs) acc)))))
+
+(defun al-merge (a b)
+  "Return a new alist with a merge of alists, A and B.
+In this case, the last writer wins, which is B."
+  (al-reduce a #'al-set b))
+
+(provide 'al)
+;;; al.el ends here