[racket-dev] racket/stream

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Wed Mar 16 10:49:37 EDT 2011

Looks great to me.


On Wed, Mar 16, 2011 at 9:04 AM, Matthew Flatt <mflatt at cs.utah.edu> wrote:
> Here's a proposal for cleaning up `racket/stream'.
> The first part of the proposal is terminology:
>  * A _sequence_ is any kind of input to `for'. That is, it's a generic
>   ordered collection of elements that supports iteration through the
>   elements.
>   Sequences include lists, vectors, and input ports.
>   All generators (as defined next) are sequences and all streams (as
>   defined further below) are sequences.
>  * A _generator_ is a stateful object that encapsulates a sequence and
>   produces the next element on demand.
>   Generators include the result of `generator' and input ports. (The
>   `generator?' predicate currently returns false for input ports, but
>   that could change.)
>   All generators are sequences. (Currently, the result of the
>   `generator' form cannot be used directly as a sequence, but part of
>   this proposal is to fix that.)
>   Not all sequences are generators. For example, a list is a sequence,
>   but it is not a generator. It's easy to create a generator from any
>   sequence by creating a stateful object that encapsulates a
>   continuation of a sequence traversal.
>  * A _stream_ is a functional object with operations `stream-first' for
>   getting the first element and `stream-rest' for getting the rest of
>   the elements. That is, for a given stream value, applying
>   `stream-first' multiple times keeps returning the first value of the
>   stream, and `stream-rest' multiple times gives back the same rest of
>   the stream.
>   Streams include lists and lazy lists as produced by `stream-cons'
>   (i.e., the usual one instead of the one currently exported by
>   `racket/stream').
>   All streams are sequences.
>   Not all sequences are streams. For example, an input port is a
>   sequence but not a stream. It's easy to define a stream in terms of
>   a sequence by lazily extracting elements from the sequence and
>   caching them.
>   A vector can be a stream only if streams are allowed to be mutable.
>   That is, `stream-first' always returns the first element of a
>   stream, and since the first element of a vector can change,
>   `stream-first' of a vector can change. Alternatively, streams could
>   be defined as immutable, in which case a vector is not a stream
>   (although a stream could be generated from a vector).
> The renaming of `racket/sequence' to `racket/stream' seems to have been
> an attempt to use the word "stream" for the most general case. But
> "stream" has an established functional meaning the Lisp world, while
> "sequence" is already set up in Racket to be the more general case. The
> above notion of "generator" is also consistent with existing uses, as
> far as I can tell. I haven't been able to find a widely used term for a
> supertype of "stream" and "generator", and I think "sequence" works as
> well as anything, but let me know if I've overlooked a term that we
> should be using.
> Based on the above terminology, here's the programming part of the
> proposal:
>  1. Rename `racket/stream' and its bindings back to "sequence".
>  2. Introduce a `racket/stream' library that provides stream versions
>    (typically lazy) of the current operations, including the expected
>    lazy `stream-cons' form.
>  3. Make streams work as sequences, and fill in various conversions
>    like `for/stream' for lazily building a stream and `in-stream' to
>    potentially speed up stream iteration via `for'.
> Adding streams as sequences effectively introduces a simpler
> representation for sequences that fit neatly into the stream protocol.
> Along those lines, operations like `sequence-append' can detect when
> all arguments are streams and defer to `stream-append' to keep any
> representation benefits intact.
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