Improve seq-concatenate for new sequence types

Use the new `seq-into-sequence' in seqs passed to `seq-concatenate' to
ensure that concatenation happens on sequences only.  This makes it
possible to use `seq-concatenate' for new types of seqs.

* lisp/emacs-lisp/seq.el (seq-into-sequence, seq-concatenate): New
function used in `seq-concatenate'.
* test/automated/seq-tests.el (test-seq-into-sequence): New unit test
for seq-into-sequence.

1 files changed, 12 insertions, 1 deletions

diff --git a/lisp/emacs-lisp/seq.el b/lisp/emacs-lisp/seq.el
index d4a9139758d..a17b0a8f1b9 100644
--- a/lisp/emacs-lisp/seq.el
+++ b/lisp/emacs-lisp/seq.el
@@ -48,6 +48,7 @@
 ;; - `seq-do'
 ;; - `seq-p'
 ;; - `seq-subseq'
+;; - `seq-into-sequence'
 ;; - `seq-copy'
 ;; - `seq-into'
 ;;
@@ -200,7 +201,17 @@ The result is a sequence of the same type as SEQ."
 TYPE must be one of following symbols: vector, string or list.
 
 \n(fn TYPE SEQUENCE...)"
-  (apply #'cl-concatenate type seqs))
+  (apply #'cl-concatenate type (seq-map #'seq-into-sequence seqs)))
+
+(cl-defgeneric seq-into-sequence (seq)
+  "Convert SEQ into a sequence.
+
+The default implementation is to signal an error if SEQ is not a
+sequence, specific functions should be implemented for new types
+of seq."
+  (unless (sequencep seq)
+    (error "Cannot convert %S into a sequence" seq))
+  seq)
 
 (cl-defgeneric seq-into (seq type)
   "Convert the sequence SEQ into a sequence of type TYPE.