[elpa] externals/stream f384660 22/36: Add systematic tests against bogus element generation

[Stefan Monnier]

branch: externals/stream
commit f384660db02ddc4e97f84f30557529ae0a3aea75
Author: Michael Heerdegen <michael_heerdegen@web.de>
Commit: Michael Heerdegen <michael_heerdegen@web.de>

    Add systematic tests against bogus element generation
    
    Also add a note about this problem in the header and how to avoid it.
---
 stream.el             | 13 ++++++++++++-
 tests/stream-tests.el | 38 ++++++++++++++++++++++++++++++++------
 2 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/stream.el b/stream.el
index 9954fc8..ef19918 100644
--- a/stream.el
+++ b/stream.el
@@ -4,7 +4,7 @@
 
 ;; Author: Nicolas Petton <nicolas@petton.fr>
 ;; Keywords: stream, laziness, sequences
-;; Version: 2.2.1
+;; Version: 2.2.2
 ;; Package-Requires: ((emacs "25"))
 ;; Package: stream
 
@@ -49,6 +49,17 @@
 ;; (defun fib (a b)
 ;;  (stream-cons a (fib b (+ a b))))
 ;; (fib 0 1)
+;;
+;; A note for developers: Please make sure to implement functions that
+;; process streams (build new streams out of given streams) in a way
+;; that no new elements in any argument stream are generated.  This is
+;; most likely an error since it changes the argument stream.  For
+;; example, a common error is to call `stream-empty-p' on an input
+;; stream and build the stream to return depending on the result.
+;; Instead, delay such tests until elements are requested from the
+;; resulting stream.  A way to achieve this is to wrap such tests into
+;; `stream-make' or `stream-delay'.  See the implementations of
+;; `stream-append' or `seq-drop-while' for example.
 
 ;;; Code:
 
diff --git a/tests/stream-tests.el b/tests/stream-tests.el
index 31c3530..e79c3ef 100644
--- a/tests/stream-tests.el
+++ b/tests/stream-tests.el
@@ -234,12 +234,6 @@
   (should (= (seq-length (seq-subseq (stream-range 2 10) 1 3)) 2))
   (should (= (seq-elt (seq-subseq (stream-range 2 10) 1 3) 1) 4)))
 
-(ert-deftest stream-seq-map-should-not-consume-stream-elements ()
-  (let* (consumed
-         (stream (stream-cons (setq consumed t) (stream-empty))))
-    (seq-map #'identity stream)
-    (should-not consumed)))
-
 (ert-deftest stream-pop-test ()
   (let* ((str (stream '(1 2 3)))
          (first (stream-pop str))
@@ -271,5 +265,37 @@
                                  (stream (list 5 6 7 8 9))))))
                  (list 1 2 3 4 5 6 7 8 9))))
 
+;; Tests whether calling stream processing functions ("transducers")
+;; doesn't generate elements from argument streams
+
+(defvar this-delayed-stream-function nil)
+
+(defun make-delayed-test-stream ()
+  (stream-make
+   (cons (prog1 1 (error "`%s' not completely delayed" 
this-delayed-stream-function))
+         (make-delayed-test-stream))))
+
+(defmacro deftest-for-delayed-evaluation (call)
+  (let ((function (car call)))
+    `(ert-deftest ,(intern (concat (symbol-name function) "-delayed-test")) ()
+       (let ((this-delayed-stream-function ',function))
+         (should (prog1 t ,call))))))
+
+(deftest-for-delayed-evaluation (streamp        (make-delayed-test-stream)))
+(deftest-for-delayed-evaluation (seqp           (make-delayed-test-stream)))
+(deftest-for-delayed-evaluation (stream-append  (make-delayed-test-stream) 
(make-delayed-test-stream)))
+(deftest-for-delayed-evaluation (seq-take (make-delayed-test-stream) 2))
+(deftest-for-delayed-evaluation (seq-drop (make-delayed-test-stream) 2))
+(deftest-for-delayed-evaluation (seq-take-while #'numberp 
(make-delayed-test-stream)))
+(deftest-for-delayed-evaluation (seq-take-until #'numberp 
(make-delayed-test-stream)))
+(deftest-for-delayed-evaluation (seq-map #'identity 
(make-delayed-test-stream)))
+(deftest-for-delayed-evaluation (seq-filter #'cl-evenp 
(make-delayed-test-stream)))
+(deftest-for-delayed-evaluation (stream-delay (make-delayed-test-stream)))
+(deftest-for-delayed-evaluation (seq-copy (make-delayed-test-stream)))
+(deftest-for-delayed-evaluation (seq-subseq (make-delayed-test-stream) 2))
+(deftest-for-delayed-evaluation (stream-scan #'* 1 (make-delayed-test-stream)))
+(deftest-for-delayed-evaluation (stream-concatenate (stream (list 
(make-delayed-test-stream)
+                                                                  
(make-delayed-test-stream)))))
+
 (provide 'stream-tests)
 ;;; stream-tests.el ends here