Add some more basic stream operations

diff --git a/packages/stream/stream.el b/packages/stream/stream.el
index 22cecac25df1eb06bf29b7480defc6eef10cba17..62eb3b6a5f5a291364068e1ede2f794c421c016f 100644 (file)
--- a/packages/stream/stream.el
+++ b/packages/stream/stream.el
@@ -333,6 +333,51 @@ calling this function."
 (cl-defmethod seq-copy ((stream stream))
   "Return a shallow copy of STREAM."
   (stream-delay stream))
+\f
+
+;;; More stream operations
+
+(defun stream-scan (function init stream)
+  "Return a stream of successive reduced values for STREAM.
+
+If the elements of a stream s are s_1, s_2, ..., the elements
+S_1, S_2, ... of the stream returned by \(stream-scan f init s\)
+are defined recursively by
+
+  S_1     = init
+  S_(n+1) = (funcall f S_n s_n)
+
+as long as s_n exists.
+
+Example:
+
+   (stream-scan #'* 1 (stream-range 1))
+
+returns a stream of the factorials."
+  (let ((res init))
+    (stream-cons
+     res
+     (seq-map (lambda (el) (setq res (funcall function res el)))
+              stream))))
+
+(defun stream-flush (stream)
+  "Request all elements from STREAM in order for side effects only."
+  (while (not (stream-empty-p stream))
+    (cl-callf stream-rest stream)))
+
+(defun stream-iterate-function (function value)
+  "Return a stream of repeated applications of FUNCTION to VALUE.
+The returned stream starts with VALUE.  Any successive element
+will be found by calling FUNCTION on the preceding element."
+  (stream-cons
+   value
+   (stream-iterate-function function (funcall function value))))
+
+(defun stream-concatenate (stream-of-streams)
+  "Concatenate all streams in STREAM-OF-STREAMS and return the result.
+All elements in STREAM-OF-STREAMS must be streams.  The result is
+a stream."
+  (seq-reduce #'stream-append stream-of-streams (stream-empty)))
 
 (defun stream-of-directory-files-1 (directory &optional nosort recurse follow-links)
   "Helper for `stream-of-directory-files'."

diff --git a/packages/stream/tests/stream-tests.el b/packages/stream/tests/stream-tests.el
index 23a54b5ec628c264ceba52c2ee20adb474766a0d..16b5756ea95407e04f4ce91a889110378d5903bc 100644 (file)
--- a/packages/stream/tests/stream-tests.el
+++ b/packages/stream/tests/stream-tests.el
@@ -242,5 +242,28 @@
     (should (= 2 (stream-first str)))
     (should (null (stream-pop stream-empty)))))
 
+(ert-deftest stream-scan-test ()
+  (should (eq (seq-elt (stream-scan #'* 1 (stream-range 1)) 4) 24)))
+
+(ert-deftest stream-flush-test ()
+  (should (let* ((times 0)
+                 (count (lambda () (cl-incf times))))
+            (letrec ((make-test-stream (lambda () (stream-cons (progn (funcall count) nil)
+                                                          (funcall make-test-stream)))))
+              (stream-flush (seq-take (funcall make-test-stream) 5))
+              (eq times 5)))))
+
+(ert-deftest stream-iterate-function-test ()
+  (should (equal (list 0 1 2) (seq-into-sequence (seq-take (stream-iterate-function #'1+ 0) 3)))))
+
+(ert-deftest stream-concatenate-test ()
+  (should (equal (seq-into-sequence
+                  (stream-concatenate
+                   (stream (list (stream (list 1 2 3))
+                                 (stream (list))
+                                 (stream (list 4))
+                                 (stream (list 5 6 7 8 9))))))
+                 (list 1 2 3 4 5 6 7 8 9))))
+
 (provide 'stream-tests)
 ;;; stream-tests.el ends here