Support multiple values in stream-cons + use stream-cons in stream operations #4554
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This pull request has been mentioned on Racket Discussions. There might be relevant details there: https://racket.discourse.group/t/stream-filter-not-in-constant-space/1643/19 |
The stream protocol and stream operations support multiple values.
However, stream-cons does not. The consequences are:
- Multiple values in streams can't be easily used by users.
- Internal stream operations are implemented in a convoluted way
to support multiple values, which is error-prone.
There are many mistakes in the current implementation,
including but not limited to:
- Incorrect laziness
- Linear time memory consumption on operations that should take
constant time
See
https://racket.discourse.group/t/stream-filter-not-in-constant-space/1643
for the discussion.
This PR adds a support of multiple values to stream-cons.
We then utilize this stream-cons on the stream operations
to fix the issues mentioned above.
Note that there are still known deficiencies in the current
implementation. These deficiencies are not degradation, however, because
SRFI-41 also has them. Furthermore, the data definition of streams in
Racket makes it difficult to fix these deficiencies without a large
refactoring, so I would like to postpone fixing these issues
to the next PRs.
See https://srfi-email.schemers.org/srfi-41/msg/21844204/
for the discussion.
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Feb 14, 2023
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This pull request has been mentioned on Racket Discussions. There might be relevant details there: https://racket.discourse.group/t/stream-filter-not-in-constant-space/1643/21 |
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This PR supports multiple values in
stream-cons. As a result,stream,stream*, andfor/streamalso support multiple values.streamandstream*patterns are also adjusted to allow matching against multiple-valued elements.We then switch stream operations which need to handle multiple values to use
stream-cons, both simplifying the implementation and making these operations properly memoizing at the same time. The switch also fixes the non-constant space problem raised in https://racket.discourse.group/t/stream-filter-not-in-constant-space/1643.