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Iterator adaptors such as .filter, .flat_map, etc. are all defined with no reference to/knowledge of the iterators they work on: http://doc.rust-lang.org/std/it

by gue5t 11y ago

Iterator adaptors such as .filter, .flat_map, etc. are all defined with no reference to/knowledge of the iterators they work on: http://doc.rust-lang.org/std/iter/trait.Iterator.html http://doc.rust-lang.org/std/iter/trait.Iterator.html They take a stream of values (i.e., an iterator, which is really only enough state to produce the stream, but not an actual collection of the values produced by that stream) and give you a different stream back, e.g. one that looks like the original stream with a function applied to each element or certain elements removed. Rust iterators are "lazy", and iterator adaptors maintain this property.

kybernetikos 11y ago
They also assume that they can always draw from their source iterator if they are called on to produce a value, which means that anything you write for them will not be reusable in scenarios where this isn't true. Transducers don't have this problem.
- gue5t 11y ago
  Can you describe some situations in which this problem comes up? I'm having a hard time imagining when I would want/expect an iterator adaptor that does something useful on an empty/finished input.
  - kybernetikos 11y ago
    It's not so much empty/finished, it's that values are produced at particular times, perhaps as the user clicks the mouse, or as messages come in from the network or over stdin. Maybe you're writing some web page analytics software. Let's say you've got a stream of values representing the (x,y) positions of mouse clicks, you want to filter out any that are more than a predefined distance from a particular point, then remove any that occur 200ms or less than the last one, then every time you have 10 of them you want to post them to the server. How would you do this with iterator adaptors? With transducers, the exact same filter that removes points further than a particular distance away in the event stream can be used on any source (e.g. an iterator).

by gue5t 11y ago

kybernetikos 11y ago

They also assume that they can always draw from their source iterator if they are called on to produce a value, which means that anything you write for them will not be reusable in scenarios where this isn't true. Transducers don't have this problem.

gue5t 11y ago
Can you describe some situations in which this problem comes up? I'm having a hard time imagining when I would want/expect an iterator adaptor that does something useful on an empty/finished input.
- kybernetikos 11y ago
  It's not so much empty/finished, it's that values are produced at particular times, perhaps as the user clicks the mouse, or as messages come in from the network or over stdin. Maybe you're writing some web page analytics software. Let's say you've got a stream of values representing the (x,y) positions of mouse clicks, you want to filter out any that are more than a predefined distance from a particular point, then remove any that occur 200ms or less than the last one, then every time you have 10 of them you want to post them to the server. How would you do this with iterator adaptors? With transducers, the exact same filter that removes points further than a particular distance away in the event stream can be used on any source (e.g. an iterator).