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+;;; tuple.el --- Tuple API for Elisp -*- lexical-binding: t -*-
+
+;; Author: William Carroll <wpcarro@gmail.com>
+;; Version: 0.0.1
+;; URL: https://git.wpcarro.dev/wpcarro/briefcase
+;; Package-Requires: ((emacs "25.1"))
+
+;;; Commentary:
+;; Work with cons cells with two elements with a familiar API for those who have
+;; worked with tuples before.
+
+;;; Code:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(cl-defstruct tuple first second)
+
+;; Create
+(defun tuple-new ()
+  "Return an empty tuple."
+  (make-tuple :first nil
+              :second nil))
+
+(defun tuple-from (a b)
+  "Return a new tuple from A and B."
+  (make-tuple :first a
+              :second b))
+
+(defun tuple-from-dotted (dp)
+  "Convert dotted pair, DP, into a tuple."
+  (tuple-from (car dp) (cdr dp)))
+
+;; Read
+(defun tuple-first (pair)
+  "Return the first element of PAIR."
+  (tuple-first pair))
+
+(defun tuple-second (pair)
+  "Return the second element of PAIR."
+  (tuple-second pair))
+
+;; Update
+(defun tuple-map-each (f g pair)
+  "Apply F to first, G to second in PAIR."
+  (->> pair
+       (tuple-map-first f)
+       (tuple-map-second g)))
+
+(defun tuple-map (f pair)
+  "Apply F to PAIR."
+  (let ((pair-copy (copy-tuple pair)))
+    (funcall f pair-copy)))
+
+(defun tuple-map-first (f pair)
+  "Apply function F to the first element of PAIR."
+  (let ((pair-copy (copy-tuple pair)))
+    (setf (tuple-first pair-copy) (funcall f (tuple-first pair-copy)))
+    pair-copy))
+
+(defun tuple-map-second (f pair)
+  "Apply function F to the second element of PAIR."
+  (let ((pair-copy (copy-tuple pair)))
+    (setf (tuple-second pair-copy) (funcall f (tuple-second pair-copy)))
+    pair-copy))
+
+(defun tuple-set-first (a pair)
+  "Return a new tuple with the first element set as A in PAIR."
+  (tuple-map-first (lambda (_) a) pair))
+
+(defun tuple-set-second (b pair)
+  "Return a new tuple with the second element set as B in PAIR."
+  (tuple-map-second (lambda (_) b) pair))
+
+;; Delete
+(defun tuple-delete-first (pair)
+  "Return PAIR with the first element set to nil."
+  (tuple-set-first nil pair))
+
+(defun tuple-delete-second (pair)
+  "Return PAIR with the second element set to nil."
+  (tuple-set-second nil pair))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Predicates
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defun tuple-instance? (x)
+  "Return t if X is a tuple."
+  (tuple-p x))
+
+(provide 'tuple)
+;;; tuple.el ends here