Make seq.el more extensible by using cl-defmethod

* lisp/emacs-lisp/seq.el: Define seq.el functions using cl-defmethod to
make it easier to extend seq.el with new "seq types".
* test/automated/seq-tests.el (test-setf-seq-elt): New test.
* lisp/emacs-lisp/cl-extra.el (cl-subseq): Move back the definition of
subseq in cl-extra.el, and use it in seq.el.

3 files changed, 224 insertions, 219 deletions

diff --git a/lisp/emacs-lisp/cl-extra.el b/lisp/emacs-lisp/cl-extra.el
index 8ed50f4f530..90ca531ae7a 100644
--- a/lisp/emacs-lisp/cl-extra.el
+++ b/lisp/emacs-lisp/cl-extra.el
@@ -38,7 +38,6 @@
 ;;; Code:
 
 (require 'cl-lib)
-(require 'seq)
 
 ;;; Type coercion.
 
@@ -520,13 +519,32 @@ This sets the values of: `cl-most-positive-float', `cl-most-negative-float',
 If END is omitted, it defaults to the length of the sequence.
 If START or END is negative, it counts from the end.
 Signal an error if START or END are outside of the sequence (i.e
-too large if positive or too small if negative)"
+too large if positive or too small if negative)."
   (declare (gv-setter
             (lambda (new)
               (macroexp-let2 nil new new
                 `(progn (cl-replace ,seq ,new :start1 ,start :end1 ,end)
                         ,new)))))
-  (seq-subseq seq start end))
+  (cond ((or (stringp seq) (vectorp seq)) (substring seq start end))
+        ((listp seq)
+         (let (len
+               (errtext (format "Bad bounding indices: %s, %s" start end)))
+           (and end (< end 0) (setq end (+ end (setq len (seq-length seq)))))
+           (if (< start 0) (setq start (+ start (or len (setq len (seq-length seq))))))
+           (unless (>= start 0)
+             (error "%s" errtext))
+           (when (> start 0)
+             (setq seq (nthcdr (1- start) seq))
+             (or seq (error "%s" errtext))
+             (setq seq (cdr seq)))
+           (if end
+               (let ((res nil))
+                 (while (and (>= (setq end (1- end)) start) seq)
+                   (push (pop seq) res))
+                 (or (= (1+ end) start) (error "%s" errtext))
+                 (nreverse res))
+             (seq-copy seq))))
+        (t (error "Unsupported sequence: %s" seq))))
 
 ;;;###autoload
 (defalias 'cl-concatenate #'seq-concatenate
diff --git a/lisp/emacs-lisp/seq.el b/lisp/emacs-lisp/seq.el
index 038b20e3b5e..f9e0e9c0fa8 100644
--- a/lisp/emacs-lisp/seq.el
+++ b/lisp/emacs-lisp/seq.el
@@ -4,7 +4,7 @@
 
 ;; Author: Nicolas Petton <nicolas@petton.fr>
 ;; Keywords: sequences
-;; Version: 1.8
+;; Version: 2.0
 ;; Package: seq
 
 ;; Maintainer: emacs-devel@gnu.org
@@ -38,10 +38,26 @@
 ;; the sequence as their second argument.  All other functions take
 ;; the sequence as their first argument.
 ;;
+;; While seq.el version 1.8 is in GNU ELPA for convenience, seq.el
+;; version 2.0 requires Emacs>=25.1.
+;;
+;; seq.el can be extended to support new type of sequences.  Here are
+;; the generic functions that must be implemented by new seq types:
+;; - `seq-elt'
+;; - `seq-length'
+;; - `seq-do'
+;; - `seq-p'
+;; - `seq-subseq'
+;; - `seq-copy'
+;; - `seq-into'
+;;
 ;; All functions are tested in test/automated/seq-tests.el
 
 ;;; Code:
 
+(eval-when-compile (require 'cl-generic))
+(require 'cl-extra) ;; for cl-subseq
+
 (defmacro seq-doseq (spec &rest body)
   "Loop over a sequence.
 Similar to `dolist' but can be applied to lists, strings, and vectors.
@@ -50,91 +66,163 @@ Evaluate BODY with VAR bound to each element of SEQ, in turn.
 
 \(fn (VAR SEQ) BODY...)"
   (declare (indent 1) (debug ((symbolp form &optional form) body)))
-  (let ((length (make-symbol "length"))
-        (seq (make-symbol "seq"))
-        (index (make-symbol "index")))
-    `(let* ((,seq ,(cadr spec))
-            (,length (if (listp ,seq) nil (seq-length ,seq)))
-            (,index (if ,length 0 ,seq)))
-       (while (if ,length
-                  (< ,index ,length)
-                (consp ,index))
-         (let ((,(car spec) (if ,length
-                                (prog1 (seq-elt ,seq ,index)
-                                  (setq ,index (+ ,index 1)))
-                              (pop ,index))))
-           ,@body)))))
-
-(if (fboundp 'pcase-defmacro)
-    ;; Implementation of `seq-let' based on a `pcase'
-    ;; pattern. Requires Emacs>=25.1.
-    (progn
-      (pcase-defmacro seq (&rest args)
-        "pcase pattern matching sequence elements.
+  `(seq-do (lambda (,(car spec))
+             ,@body)
+           ,(cadr spec)))
+
+(pcase-defmacro seq (&rest args)
+  "pcase pattern matching sequence elements.
 Matches if the object is a sequence (list, string or vector), and
 binds each element of ARGS to the corresponding element of the
 sequence."
-        `(and (pred seq-p)
-              ,@(seq--make-pcase-bindings args)))
+  `(and (pred seq-p)
+        ,@(seq--make-pcase-bindings args)))
 
-      (defmacro seq-let (args seq &rest body)
-        "Bind the variables in ARGS to the elements of SEQ then evaluate BODY.
+(defmacro seq-let (args seq &rest body)
+  "Bind the variables in ARGS to the elements of SEQ then evaluate BODY.
 
 ARGS can also include the `&rest' marker followed by a variable
 name to be bound to the rest of SEQ."
-        (declare (indent 2) (debug t))
-        `(pcase-let ((,(seq--make-pcase-patterns args) ,seq))
-           ,@body)))
+  (declare (indent 2) (debug t))
+  `(pcase-let ((,(seq--make-pcase-patterns args) ,seq))
+     ,@body))
+
+
+;;; Basic seq functions that have to be implemented by new seq types
+(cl-defgeneric seq-elt (seq n)
+  "Return the element of SEQ at index N."
+  (elt seq n))
+
+;; Default gv setters for `seq-elt'.
+;; It can be a good idea for new sequence impelentations to provide a
+;; "gv-setter" for `seq-elt'.
+(cl-defmethod (setf seq-elt) (store (seq array) n)
+  (aset seq n store))
+
+(cl-defmethod (setf seq-elt) (store (seq cons) n)
+  (setcar (nthcdr n seq) store))
+
+(cl-defgeneric seq-length (seq)
+  "Return the length of the sequence SEQ."
+  (length seq))
+
+(cl-defgeneric seq-do (function seq)
+  "Apply FUNCTION to each element of SEQ, presumably for side effects.
+Return SEQ."
+  (mapc function seq))
 
-  ;; Implementation of `seq-let' compatible with Emacs<25.1.
-  (defmacro seq-let (args seq &rest body)
-    "Bind the variables in ARGS to the elements of SEQ then evaluate BODY.
+(defalias 'seq-each #'seq-do)
 
-ARGS can also include the `&rest' marker followed by a variable
-name to be bound to the rest of SEQ."
-    (declare (indent 2) (debug t))
-    (let ((seq-var (make-symbol "seq")))
-      `(let* ((,seq-var ,seq)
-              ,@(seq--make-bindings args seq-var))
-         ,@body))))
+(cl-defgeneric seq-p (seq)
+  "Return non-nil if SEQ is a sequence, nil otherwise."
+  (sequencep seq))
+
+(cl-defgeneric seq-copy (seq)
+  "Return a shallow copy of SEQ."
+  (copy-sequence seq))
+
+(cl-defgeneric seq-subseq (seq start &optional end)
+  "Return the subsequence of SEQ from START to END.
+If END is omitted, it defaults to the length of the sequence.
+If START or END is negative, it counts from the end.
+Signal an error if START or END are outside of the sequence (i.e
+too large if positive or too small if negative)."
+  (cl-subseq seq start end))
+
+
+(cl-defgeneric seq-map (function seq)
+  "Return the result of applying FUNCTION to each element of SEQ."
+  (let (result)
+    (seq-do (lambda (elt)
+              (push (funcall function elt) result))
+            seq)
+    (nreverse result)))
+
+;; faster implementation for sequences (sequencep)
+(cl-defmethod seq-map (function (seq sequence))
+  (mapcar function seq))
 
-(defun seq-drop (seq n)
+(cl-defgeneric seq-drop (seq n)
   "Return a subsequence of SEQ without its first N elements.
 The result is a sequence of the same type as SEQ.
 
 If N is a negative integer or zero, SEQ is returned."
   (if (<= n 0)
       seq
-    (if (listp seq)
-        (seq--drop-list seq n)
-      (let ((length (seq-length seq)))
-        (seq-subseq seq (min n length) length)))))
+    (let ((length (seq-length seq)))
+      (seq-subseq seq (min n length) length))))
 
-(defun seq-take (seq n)
+(cl-defgeneric seq-take (seq n)
   "Return a subsequence of SEQ with its first N elements.
 The result is a sequence of the same type as SEQ.
 
 If N is a negative integer or zero, an empty sequence is
 returned."
-  (if (listp seq)
-      (seq--take-list seq n)
-    (seq-subseq seq 0 (min (max n 0) (seq-length seq)))))
+  (seq-subseq seq 0 (min (max n 0) (seq-length seq))))
 
-(defun seq-drop-while (pred seq)
+(cl-defgeneric seq-drop-while (pred seq)
   "Return a sequence from the first element for which (PRED element) is nil in SEQ.
 The result is a sequence of the same type as SEQ."
-  (if (listp seq)
-      (seq--drop-while-list pred seq)
-    (seq-drop seq (seq--count-successive pred seq))))
+  (seq-drop seq (seq--count-successive pred seq)))
 
-(defun seq-take-while (pred seq)
+(cl-defgeneric seq-take-while (pred seq)
   "Return the successive elements for which (PRED element) is non-nil in SEQ.
 The result is a sequence of the same type as SEQ."
-  (if (listp seq)
-      (seq--take-while-list pred seq)
-    (seq-take seq (seq--count-successive pred seq))))
+  (seq-take seq (seq--count-successive pred seq)))
 
-(defun seq-filter (pred seq)
+(cl-defgeneric seq-empty-p (seq)
+  "Return non-nil if the sequence SEQ is empty, nil otherwise."
+  (= 0 (seq-length seq)))
+
+(cl-defgeneric seq-sort (pred seq)
+  "Return a sorted sequence comparing using PRED the elements of SEQ.
+The result is a sequence of the same type as SEQ."
+  (let ((result (seq-sort pred (append seq nil))))
+    (seq-into result (type-of seq))))
+
+(cl-defmethod seq-sort (pred (list list))
+  (sort (seq-copy list) pred))
+
+(cl-defgeneric seq-reverse (seq)
+  "Return the reversed shallow copy of SEQ."
+  (let ((result '()))
+    (seq-map (lambda (elt)
+               (push elt result))
+             seq)
+    (seq-into result (type-of seq))))
+
+;; faster implementation for sequences (sequencep)
+(cl-defmethod seq-reverse ((seq sequence))
+  (reverse seq))
+
+(cl-defgeneric seq-concatenate (type &rest seqs)
+  "Concatenate, into a sequence of type TYPE, the sequences SEQS.
+TYPE must be one of following symbols: vector, string or list.
+
+\n(fn TYPE SEQUENCE...)"
+  ;; Since new seq types might be defined, we need to make sure that
+  ;; all seqs are actual sequences.
+  (let ((sequences (seq-map (lambda (s)
+                              (if (sequencep s)
+                                  s
+                                (seq-into s 'list)))
+                            seqs)))
+    (pcase type
+      (`vector (apply #'vconcat sequences))
+      (`string (apply #'concat sequences))
+      (`list (apply #'append (append sequences '(nil))))
+      (_ (error "Not a sequence type name: %S" type)))))
+
+(cl-defgeneric seq-into (seq type)
+  "Convert the sequence SEQ into a sequence of type TYPE.
+TYPE can be one of the following symbols: vector, string or list."
+  (pcase type
+    (`vector (vconcat seq))
+    (`string (concat seq))
+    (`list (append seq nil))
+    (_ (error "Not a sequence type name: %S" type))))
+
+(cl-defgeneric seq-filter (pred seq)
   "Return a list of all the elements for which (PRED element) is non-nil in SEQ."
   (let ((exclude (make-symbol "exclude")))
     (delq exclude (seq-map (lambda (elt)
@@ -143,12 +231,12 @@ The result is a sequence of the same type as SEQ."
                                exclude))
                            seq))))
 
-(defun seq-remove (pred seq)
+(cl-defgeneric seq-remove (pred seq)
   "Return a list of all the elements for which (PRED element) is nil in SEQ."
   (seq-filter (lambda (elt) (not (funcall pred elt)))
               seq))
 
-(defun seq-reduce (function seq initial-value)
+(cl-defgeneric seq-reduce (function seq initial-value)
   "Reduce the function FUNCTION across SEQ, starting with INITIAL-VALUE.
 
 Return the result of calling FUNCTION with INITIAL-VALUE and the
@@ -164,7 +252,7 @@ If SEQ is empty, return INITIAL-VALUE and FUNCTION is not called."
         (setq acc (funcall function acc elt)))
       acc)))
 
-(defun seq-some-p (pred seq)
+(cl-defgeneric seq-some-p (pred seq)
   "Return any element for which (PRED element) is non-nil in SEQ, nil otherwise."
   (catch 'seq--break
     (seq-doseq (elt seq)
@@ -172,7 +260,7 @@ If SEQ is empty, return INITIAL-VALUE and FUNCTION is not called."
         (throw 'seq--break elt)))
     nil))
 
-(defun seq-every-p (pred seq)
+(cl-defgeneric seq-every-p (pred seq)
   "Return non-nil if (PRED element) is non-nil for all elements of the sequence SEQ."
   (catch 'seq--break
     (seq-doseq (elt seq)
@@ -180,7 +268,7 @@ If SEQ is empty, return INITIAL-VALUE and FUNCTION is not called."
           (throw 'seq--break nil)))
     t))
 
-(defun seq-count (pred seq)
+(cl-defgeneric seq-count (pred seq)
   "Return the number of elements for which (PRED element) is non-nil in SEQ."
   (let ((count 0))
     (seq-doseq (elt seq)
@@ -188,28 +276,14 @@ If SEQ is empty, return INITIAL-VALUE and FUNCTION is not called."
         (setq count (+ 1 count))))
     count))
 
-(defun seq-empty-p (seq)
-  "Return non-nil if the sequence SEQ is empty, nil otherwise."
-  (if (listp seq)
-      (null seq)
-    (= 0 (seq-length seq))))
-
-(defun seq-sort (pred seq)
-  "Return a sorted sequence comparing using PRED the elements of SEQ.
-The result is a sequence of the same type as SEQ."
-  (if (listp seq)
-      (sort (seq-copy seq) pred)
-    (let ((result (seq-sort pred (append seq nil))))
-      (seq-into result (type-of seq)))))
-
-(defun seq-contains-p (seq elt &optional testfn)
+(cl-defgeneric seq-contains-p (seq elt &optional testfn)
   "Return the first element in SEQ that equals to ELT.
 Equality is defined by TESTFN if non-nil or by `equal' if nil."
   (seq-some-p (lambda (e)
                 (funcall (or testfn #'equal) elt e))
               seq))
 
-(defun seq-uniq (seq &optional testfn)
+(cl-defgeneric seq-uniq (seq &optional testfn)
   "Return a list of the elements of SEQ with duplicates removed.
 TESTFN is used to compare elements, or `equal' if TESTFN is nil."
   (let ((result '()))
@@ -218,51 +292,13 @@ TESTFN is used to compare elements, or `equal' if TESTFN is nil."
         (setq result (cons elt result))))
     (nreverse result)))
 
-(defun seq-subseq (seq start &optional end)
-  "Return the subsequence of SEQ from START to END.
-If END is omitted, it defaults to the length of the sequence.
-If START or END is negative, it counts from the end.
-
-Signal an error if START or END are outside of the sequence (i.e
-too large if positive or too small if negative)"
-  (cond ((or (stringp seq) (vectorp seq)) (substring seq start end))
-        ((listp seq)
-         (let (len (errtext (format "Bad bounding indices: %s, %s" start end)))
-           (and end (< end 0) (setq end (+ end (setq len (seq-length seq)))))
-           (if (< start 0) (setq start (+ start (or len (setq len (seq-length seq))))))
-           (unless (>= start 0)
-             (error "%s" errtext))
-           (when (> start 0)
-             (setq seq (nthcdr (1- start) seq))
-             (or seq (error "%s" errtext))
-             (setq seq (cdr seq)))
-           (if end
-               (let ((res nil))
-                 (while (and (>= (setq end (1- end)) start) seq)
-                   (push (pop seq) res))
-                 (or (= (1+ end) start) (error "%s" errtext))
-                 (nreverse res))
-             (seq-copy seq))))
-        (t (error "Unsupported sequence: %s" seq))))
-
-(defun seq-concatenate (type &rest seqs)
-  "Concatenate, into a sequence of type TYPE, the sequences SEQS.
-TYPE must be one of following symbols: vector, string or list.
-
-\n(fn TYPE SEQUENCE...)"
-  (pcase type
-    (`vector (apply #'vconcat seqs))
-    (`string (apply #'concat seqs))
-    (`list (apply #'append (append seqs '(nil))))
-    (_ (error "Not a sequence type name: %S" type))))
-
-(defun seq-mapcat (function seq &optional type)
+(cl-defgeneric seq-mapcat (function seq &optional type)
   "Concatenate the result of applying FUNCTION to each element of SEQ.
 The result is a sequence of type TYPE, or a list if TYPE is nil."
   (apply #'seq-concatenate (or type 'list)
          (seq-map function seq)))
 
-(defun seq-partition (seq n)
+(cl-defgeneric seq-partition (seq n)
   "Return a list of the elements of SEQ grouped into sub-sequences of length N.
 The last sequence may contain less than N elements.  If N is a
 negative integer or 0, nil is returned."
@@ -273,7 +309,7 @@ negative integer or 0, nil is returned."
         (setq seq (seq-drop seq n)))
       (nreverse result))))
 
-(defun seq-intersection (seq1 seq2 &optional testfn)
+(cl-defgeneric seq-intersection (seq1 seq2 &optional testfn)
   "Return a list of the elements that appear in both SEQ1 and SEQ2.
 Equality is defined by TESTFN if non-nil or by `equal' if nil."
   (seq-reduce (lambda (acc elt)
@@ -283,7 +319,7 @@ Equality is defined by TESTFN if non-nil or by `equal' if nil."
               (seq-reverse seq1)
               '()))
 
-(defun seq-difference (seq1 seq2 &optional testfn)
+(cl-defgeneric seq-difference (seq1 seq2 &optional testfn)
   "Return a list of the elements that appear in SEQ1 but not in SEQ2.
 Equality is defined by TESTFN if non-nil or by `equal' if nil."
   (seq-reduce (lambda (acc elt)
@@ -293,7 +329,7 @@ Equality is defined by TESTFN if non-nil or by `equal' if nil."
               (seq-reverse seq1)
               '()))
 
-(defun seq-group-by (function seq)
+(cl-defgeneric seq-group-by (function seq)
   "Apply FUNCTION to each element of SEQ.
 Separate the elements of SEQ into an alist using the results as
 keys.  Keys are compared using `equal'."
@@ -308,70 +344,16 @@ keys.  Keys are compared using `equal'."
    (seq-reverse seq)
    nil))
 
-(defalias 'seq-reverse
-  (if (ignore-errors (reverse [1 2]))
-      #'reverse
-    (lambda (seq)
-      "Return the reversed copy of list, vector, or string SEQ.
-See also the function `nreverse', which is used more often."
-      (let ((result '()))
-        (seq-map (lambda (elt) (push elt result))
-                 seq)
-        (if (listp seq)
-            result
-          (seq-into result (type-of seq)))))))
-
-(defun seq-into (seq type)
-  "Convert the sequence SEQ into a sequence of type TYPE.
-TYPE can be one of the following symbols: vector, string or list."
-  (pcase type
-    (`vector (vconcat seq))
-    (`string (concat seq))
-    (`list (append seq nil))
-    (_ (error "Not a sequence type name: %S" type))))
-
-(defun seq-min (seq)
+(cl-defgeneric seq-min (seq)
   "Return the smallest element of SEQ.
 SEQ must be a sequence of numbers or markers."
   (apply #'min (seq-into seq 'list)))
 
-(defun seq-max (seq)
-    "Return the largest element of SEQ.
+(cl-defgeneric seq-max (seq)
+  "Return the largest element of SEQ.
 SEQ must be a sequence of numbers or markers."
   (apply #'max (seq-into seq 'list)))
 
-(defun seq--drop-list (list n)
-  "Return a list from LIST without its first N elements.
-This is an optimization for lists in `seq-drop'."
-  (while (and list (> n 0))
-    (setq list (cdr list)
-          n (1- n)))
-  list)
-
-(defun seq--take-list (list n)
-  "Return a list from LIST made of its first N elements.
-This is an optimization for lists in `seq-take'."
-  (let ((result '()))
-    (while (and list (> n 0))
-      (setq n (1- n))
-      (push (pop list) result))
-    (nreverse result)))
-
-(defun seq--drop-while-list (pred list)
-  "Return a list from the first element for which (PRED element) is nil in LIST.
-This is an optimization for lists in `seq-drop-while'."
-  (while (and list (funcall pred (car list)))
-    (setq list (cdr list)))
-  list)
-
-(defun seq--take-while-list (pred list)
-  "Return the successive elements for which (PRED element) is non-nil in LIST.
-This is an optimization for lists in `seq-take-while'."
-  (let ((result '()))
-    (while (and list (funcall pred (car list)))
-      (push (pop list) result))
-    (nreverse result)))
-
 (defun seq--count-successive (pred seq)
   "Return the number of successive elements for which (PRED element) is non-nil in SEQ."
   (let ((n 0)
@@ -408,52 +390,52 @@ This is an optimization for lists in `seq-take-while'."
                      elt))
                  args)))
 
-;; Helper function for the Backward-compatible version of `seq-let'
-;; for Emacs<25.1.
-(defun seq--make-bindings (args seq &optional bindings)
-  "Return a list of bindings of the variables in ARGS to the elements of a sequence.
-if BINDINGS is non-nil, append new bindings to it, and return
-BINDINGS."
-  (let ((index 0)
-        (rest-marker nil))
-    (seq-doseq (name args)
-      (unless rest-marker
-        (pcase name
-          ((pred seq-p)
-           (setq bindings (seq--make-bindings (seq--elt-safe args index)
-                                              `(seq--elt-safe ,seq ,index)
-                                              bindings)))
-          (`&rest
-           (progn (push `(,(seq--elt-safe args (1+ index))
-                          (seq-drop ,seq ,index))
-                        bindings)
-                  (setq rest-marker t)))
-          (_
-           (push `(,name (seq--elt-safe ,seq ,index)) bindings))))
-      (setq index (1+ index)))
-    bindings))
-
+;; TODO: make public?
 (defun seq--elt-safe (seq n)
   "Return element of SEQ at the index N.
 If no element is found, return nil."
-  (when (or (listp seq)
-            (and (sequencep seq)
-                 (> (seq-length seq) n)))
-    (seq-elt seq n)))
+  (ignore-errors (seq-elt seq n)))
+
+
+;;; Optimized implementations for lists
+
+(cl-defmethod seq-drop ((list list) n)
+  "Optimized implementation of `seq-drop' for lists."
+  (while (and list (> n 0))
+    (setq list (cdr list)
+          n (1- n)))
+  list)
+
+(cl-defmethod seq-take ((list list) n)
+  "Optimized implementation of `seq-take' for lists."
+  (let ((result '()))
+    (while (and list (> n 0))
+      (setq n (1- n))
+      (push (pop list) result))
+    (nreverse result)))
+
+(cl-defmethod seq-drop-while (pred (list list))
+  "Optimized implementation of `seq-drop-while' for lists"
+  (while (and list (funcall pred (car list)))
+    (setq list (cdr list)))
+  list)
+
+(cl-defmethod seq-drop-while (pred (list list))
+  "Optimized implementation of `seq-drop-while' for lists"
+  (while (and list (funcall pred (car list)))
+    (setq list (cdr list)))
+  list)
+
+(cl-defmethod seq-empty-p ((list list))
+  "Optimized implementation of `seq-empty-p' for lists."
+  (null list))
+
 
 (defun seq--activate-font-lock-keywords ()
   "Activate font-lock keywords for some symbols defined in seq."
   (font-lock-add-keywords 'emacs-lisp-mode
                           '("\\<seq-doseq\\>" "\\<seq-let\\>")))
 
-(defalias 'seq-copy #'copy-sequence)
-(defalias 'seq-elt #'elt)
-(defalias 'seq-length #'length)
-(defalias 'seq-do #'mapc)
-(defalias 'seq-each #'seq-do)
-(defalias 'seq-map #'mapcar)
-(defalias 'seq-p #'sequencep)
-
 (unless (fboundp 'elisp--font-lock-flush-elisp-buffers)
   ;; In Emacs≥25, (via elisp--font-lock-flush-elisp-buffers and a few others)
   ;; we automatically highlight macros.
diff --git a/test/automated/seq-tests.el b/test/automated/seq-tests.el
index 74c0700759e..163935b5432 100644
--- a/test/automated/seq-tests.el
+++ b/test/automated/seq-tests.el
@@ -54,6 +54,11 @@ Evaluate BODY for each created sequence.
   "Return t if INTEGER is odd."
   (not (test-sequences-evenp integer)))
 
+(ert-deftest test-setf-seq-elt ()
+  (with-test-sequences (seq '(1 2 3))
+    (setf (seq-elt seq 1) 4)
+    (should (= 4 (seq-elt seq 1)))))
+
 (ert-deftest test-seq-drop ()
   (with-test-sequences (seq '(1 2 3 4))
     (should (equal (seq-drop seq 0) seq))
@@ -192,7 +197,7 @@ Evaluate BODY for each created sequence.
   (should-error (seq-subseq [] -1))
   (should-error (seq-subseq "" -1))
   (should-not (seq-subseq '() 0))
-  (should-error(seq-subseq '() 0 -1)))
+  (should-error (seq-subseq '() 0 -1)))
 
 (ert-deftest test-seq-concatenate ()
   (with-test-sequences (seq '(2 4 6))