;;; 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:
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;; Dependencies:
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(require 'macros)
(require 'dash)
(require 'tuple)
(require 'maybe)
;; 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)'.
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;; Constants
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(defconst al-enable-tests? t
"When t, run the test suite.")
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;; Library
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;; 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)
"Return the value at K in XS; otherwise, return nil.
Returns the first occurrence of K in XS since alists support multiple entries."
(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."
(maybe-some? (assoc k xs)))
(defun al-has-value? (v xs)
"Return t if XS has a value of V."
(maybe-some? (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))
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;; Tests
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(when al-enable-tests?
(prelude-assert
(equal '((2 . one)
(3 . two))
(al-map-keys #'1+
'((1 . one)
(2 . two)))))
(prelude-assert
(equal '((one . 2)
(two . 3))
(al-map-values #'1+
'((one . 1)
(two . 2))))))
;; TODO: Support test cases for the entire API.
(provide 'al)
;;; al.el ends here
