;;; struct.el --- Helpers for working with structs -*- lexical-binding: t -*-

;; Author: William Carroll <wpcarro@gmail.com>
;; Version: 0.0.1
;; Package-Requires: ((emacs "24.3"))

;;; Commentary:
;; Provides new macros for working with structs.  Also provides adapter
;; interfaces to existing struct macros, that should have more intuitive
;; interfaces.
;;
;; Sometimes `setf' just isn't enough.

;;; Code:

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;; Dependencies
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(require 'string)
(require 'dash)

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;; Library
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(defvar struct--enable-tests? t
  "When t, run the test suite defined herein.")

(defmacro struct-update (type field f xs)
  "Apply F to FIELD in XS, which is a struct of TYPE.
This is immutable."
  (let ((copier (->> type
                     symbol-name
                     (string-prepend "copy-")
                     intern))
        (accessor (->> field
                       symbol-name
                       (string-prepend (string-concat (symbol-name type) "-"))
                       intern)))
    `(let ((copy (,copier ,xs)))
       (setf (,accessor copy) (funcall ,f (,accessor copy)))
       copy)))

(defmacro struct-set (type field x xs)
  "Immutably set FIELD in XS (struct TYPE) to X."
  (let ((copier (->> type
                     symbol-name
                     (string-prepend "copy-")
                     intern))
        (accessor (->> field
                       symbol-name
                       (string-prepend (string-concat (symbol-name type) "-"))
                       intern)))
    `(let ((copy (,copier ,xs)))
       (setf (,accessor copy) ,x)
       copy)))

(defmacro struct-set! (type field x xs)
  "Set FIELD in XS (struct TYPE) to X mutably.
This is an adapter interface to `setf'."
  (let ((accessor (->> field
                       symbol-name
                       (string-prepend (string-concat (symbol-name type) "-"))
                       intern)))
    `(progn
       (setf (,accessor ,xs) ,x)
       ,xs)))

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;; Tests
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(when struct--enable-tests?
  (cl-defstruct dummy name age)
  (defvar struct--test-dummy (make-dummy :name "Roofus" :age 19))
  (struct-set! dummy name "Doofus" struct--test-dummy)
  (prelude-assert (string= "Doofus" (dummy-name struct--test-dummy)))
  (let ((result (struct-set dummy name "Shoofus" struct--test-dummy)))
    ;; Test the immutability of `struct-set'
    (prelude-assert (string= "Doofus" (dummy-name struct--test-dummy)))
    (prelude-assert (string= "Shoofus" (dummy-name result)))))

(provide 'struct)
;;; struct.el ends here