Lazy list in programing languages, python iterator, JavaScript iterable iterator generator.
lazy list
- i think lazy list is a bad feature in programing languages, e.g. python iterator and js iterable iterator generator.
- because, they make list complicated.
- but i think the idea is useful for stream.
- So i thought, languages should not have lazy list but a dedicated stream data type.
- do i make sense, or, when actually done, my stream datatype is not much different than iterator as exist in python js?
programing language design, lazy list
- i think lazy list is a bad feature in programing languages, e.g. python iterator and js iterable iterator generator.
- because, they make list complicated.
- but i think the idea is useful for stream.
- So i thought, languages should not have lazy list but a dedicated stream data type.
- do i make sense, or, when actually done, my stream datatype is not much different than iterator as exist in python js?
ai answer
Yes, you make a lot of sense. Separating eager collections (like lists/arrays) from a dedicated lazy/stream abstraction is a solid design intuition, and many languages already lean into something like this—though the exact boundaries vary. Your concern about "lazy lists complicating lists" is valid: mixing laziness into a basic data structure often introduces surprises around memory, evaluation timing, debugging, and performance.
Why Lazy Lists Can Feel Bad
- Lists should be simple and predictable: A plain list/array is usually expected to be eager (fully materialized in memory, random-access, simple semantics). Adding laziness blurs this—e.g., in Haskell, the default list is lazy, which enables beautiful infinite data but can lead to space leaks, thunk build-up, and mysterious performance issues.
- Python/JS issues: Python's
listis eager; iterators/generators are separate but the ecosystem mixes them freely. You can accidentally consume an iterator twice, or turn a generator into a list unintentionally. JS iterables/generators have similar ergonomics gotchas. - Laziness everywhere adds overhead (bookkeeping for thunks) and makes reasoning about when things happen harder.
A dedicated stream type cleanly solves this by keeping lists eager/simple while providing a purpose-built tool for lazy, potentially infinite, or large/on-demand data.
How Streams Differ from Plain Iterators
Your proposed "stream" datatype is similar to existing iterators in core mechanics (both are pull-based, lazy), but a good dedicated stream abstraction usually adds higher-level value. Here's the practical difference:
| Aspect | Low-level Iterator (Python/JS style) | Dedicated Stream (e.g. Java Streams, Scala Stream/View, Haskell lazy lists) |
|---|---|---|
| Core | next() / yield, single-pass, consumable |
Builds on iterator-like pulling but with pipeline semantics |
| Composability | Manual (for loops, or itertools in Python) |
Rich methods: .map(), .filter(), .flatMap(), etc., that chain lazily |
| Laziness | Yes, but evaluation is explicit | Intermediate ops are fully lazy; only terminal op (.collect(), .forEach()) triggers work |
| Multiple passes | Usually destructive (once consumed, gone) | Some allow reuse (e.g. views); others are single-pass |
| Infinite support | Possible but clunky | Natural (e.g. infinite Fibonacci) |
| Resource mgmt | Manual (try/finally) | Often better (e.g. Java Streams can handle AutoCloseable) |
| Performance | Lightweight, minimal overhead | Slight overhead from abstraction, but enables fusion/optimization |
| Ergonomics | Low-level, imperative feel | Declarative, functional style |
Examples:
- Java Streams vs Iterators: Streams shine for declarative pipelines on collections or I/O. They're not just iterators—they're a fluent API on top. Iterators are more bare-bones and performant for simple cases.
- Scala: Has
Iterator(destructive, low-level),Stream(memoizing lazy list, can be infinite), andView(non-memoizing lazy view). They serve different niches. - Python: Generators/iterators are close to what you want, but lack built-in rich chaining (you use
itertoolsor libraries likemore-itertools). People often build "stream-like" helpers on top. - Rust: Iterators are very powerful with zero-cost abstractions and fusion (optimizing chained operations). They feel like a strong "stream" without a separate type.
In short: A dedicated Stream type is often not much different at the implementation level from a good iterator + adapter pattern, but it feels different and is more ergonomic for common use cases (data processing pipelines, reactive-ish flows, big data). It's the API and guarantees (laziness semantics, chaining, termination) that make it valuable.
When a Dedicated Stream Wins
- Processing large/infinite data without blowing memory.
- Clean functional pipelines (
data.stream().filter(...).map(...).take(100).collect()). - Better separation of concerns:
List= random access + eager;Stream= sequential + lazy. - Easier optimization (fusion of operations) and debugging in some designs.
Downsides / Why Not Always Separate
- Extra type in the language/stdlib adds complexity.
- Many languages get 80% there with iterators + libraries (Python's generator expressions are surprisingly nice).
- For performance-critical code, raw iterators or loops are still preferred.
Your idea aligns with trends in modern languages and libraries (Reactive Streams, Kotlin Flows, etc., which add push + backpressure on top of pull). It's a good instinct—keep collections simple, make laziness explicit and powerful via streams. Many language designers have done exactly this, with varying success.
If you're designing a language or library, leaning into a first-class Stream with good syntax for infinite cases and composition would be a nice improvement over just exposing raw iterators. What language/context are you thinking about this for?
The Ills of Iterators, Generators, Iterable
- In programing language, iterators, generators, iterable, should be ban'd.
- They screw the fundamental concept of math function.
- They are not functions. They are state machine tricks, a side effect of computer engineering, like int float double long and references.
- I think SICP, racket scheme lisp, and haskell are idiotic in seeding that thought.
- Not only screw function concept, but also make things very complex.
- By the way, which lang first implemented the lazy list specifically?
- And what was the history of iterators, generators, iterable, their specific meaning at the time, in what lang?
- Need to do research on this.