Move move .emacs.d out of configs/shared

Moving all of my Emacs-related files into their own directory at the root of
this repository.

emacs/.emacs.d/wpc/vector.el

@@ -0,0 +1,81 @@
+;;; vector.el --- Working with Elisp's Vector data type -*- lexical-binding: t -*-
+;; Author: William Carroll <wpcarro@gmail.com>
+
+;;; Commentary:
+;; It might be best to think of Elisp vectors as tuples in languages like
+;; Haskell or Erlang.
+;;
+;; Not surprisingly, this API is modelled after Elixir's Tuple API.
+;;
+;; Some Elisp trivia:
+;; - "Array": Usually means vector or string.
+;; - "Sequence": Usually means list or "array" (see above).
+;;
+;; It might be a good idea to think of Array and Sequence as typeclasses in
+;; Elisp.  This is perhaps more similar to Elixir's notion of the Enum protocol.
+;;
+;; Intentionally not supporting a to-list function, because tuples can contain
+;; heterogenous types whereas lists should contain homogenous types.
+
+;;; Code:
+
+;; TODO: Consider supporting an alias named tuple for vector.
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defconst vector/enable-tests? t
+  "When t, run the tests defined herein.")
+
+;; TODO: Consider labelling variadic functions like `vector/concat*'
+;; vs. `vector/concat'.
+(defun vector/concat (&rest args)
+  "Return a new vector composed of all vectors in `ARGS'."
+  (apply #'vconcat args))
+
+;; TODO: Here's a sketch of a protocol macro being consumed.
+;; (definstance monoid vector
+;;   :empty (lambda () []))
+
+(defun vector/prepend (x xs)
+  "Add `X' to the beginning of `XS'."
+  (vector/concat `[,x] xs))
+
+(defun vector/append (x xs)
+  "Add `X' to the end of `XS'."
+  (vector/concat xs `[,x]))
+
+(defun vector/get (i xs)
+  "Return the value in `XS' at index, `I'."
+  (aref xs i))
+
+(defun vector/set (i v xs)
+  "Set index `I' to value `V' in `XS'.
+Returns a copy of `XS' with the updates."
+  (let ((copy (vconcat [] xs)))
+    (aset copy i v)
+    copy))
+
+(defun vector/set! (i v xs)
+  "Set index `I' to value `V' in `XS'.
+This function mutates XS."
+  (aset xs i v))
+
+(when vector/enable-tests?
+  (let ((xs [1 2 3])
+        (ys [1 2 3]))
+    (prelude/assert (= 1 (vector/get 0 ys)))
+    (vector/set 0 4 ys)
+    (prelude/assert (= 1 (vector/get 0 ys)))
+    (prelude/assert (= 1 (vector/get 0 xs)))
+    (vector/set! 0 4 xs)
+    (prelude/assert (= 4 (vector/get 0 xs)))))
+
+;; TODO: Decide between "remove" and "delete" as the appropriate verbs.
+;; TODO: Implement this.
+;; (defun vector/delete (i xs)
+;;   "Remove the element at `I' in `XS'.")
+
+(provide 'vector)
+;;; vector.el ends here