+1 2011/3/16 Matthew Flatt <[email protected]>: > 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. > > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/dev >
-- Jay McCarthy <[email protected]> Assistant Professor / Brigham Young University http://faculty.cs.byu.edu/~jay "The glory of God is Intelligence" - D&C 93 _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/dev
