From: "UlyssesZhan (Ulysses Zhan) via ruby-core" <ruby-core@...>
Date: 2026-10-25T02:16:45+00:00
Subject: [ruby-core:127232] [Ruby Feature#17016] Enumerable#accumulate

Issue #17016 has been updated by UlyssesZhan (Ulysses Zhan).


Personally I have stumbled upon use cases of this method for at least three times. I finally found this feature request. I forgot what I wanted to do the previous times, but the last time I wanted this feature is when I wanted to generate a sequence of binomial coefficients:

```ruby
module Enumerable
  def accumulate init, &f
    each_with_object([init]) { _2.push f.(_2.last, _1) }
  end
end

def binom n, c = n
  c.times.accumulate(1) { _1 * (n - _2) / (_2 + 1) }
end
```

----------------------------------------
Feature #17016: Enumerable#accumulate
https://bugs.ruby-lang.org/issues/17016#change-119648

* Author: parker (Parker Finch)
* Status: Open
----------------------------------------
## Proposal

UPDATE: Changed proposed method name from `#scan_left` to `#accumulate`.

Add an `#accumulate` method to `Enumerable`.

## Background

`#accumulate` is similar to `#inject`, but it accumulates the partial results that are computed. As a comparison:
```
[1, 2, 3].inject(0, &:+) => 6
[1, 2, 3].accumulate(0, &:+) => [0, 1, 3, 6]
```

Notably, the `accumulate` operation can be done lazily since it doesn't require processing the entire collection before computing a value.

I recently described `#accumulate`, and its relationship to `#inject`, more thoroughly in [this blog post](https://medium.com/building-panorama-education/scan-left-a-lazy-incremental-alternative-to-inject-f6e946f74c00).

## Reasoning
We heavily rely on the accumulate operation. We use an [event-sourcing](https://martinfowler.com/eaaDev/EventSourcing.html) pattern, which means that we are scanning over individual "events" and building up the corresponding state. We rely on the history of states and need to do this lazily (we stream events because they cannot fit in memory). Thus the scan operation is much more applicable than the inject operation.

We suspect that there are many applications that could leverage the scan operation. [This question](https://stackoverflow.com/questions/1475808/cumulative-array-sum-in-ruby) would be more easily answered by `#accumulate`. It is a natural fit for any application that needs to store the incrementally-computed values of an `#inject`, and a requirement for an application that needs to use `#inject` while maintaining laziness.

## Implementation
There is a Ruby implementation of this functionality [here](https://github.com/panorama-ed/scan_left/) and an implementation in C [here](https://github.com/ruby/ruby/pull/3078).

Update: @nobu has provided an alternative implementation [here](https://github.com/ruby/ruby/pull/1972).

## Counterarguments
Introducing a new public method is committing to maintenance going forward and expands the size of the Ruby codebase -- it should not be done lightly. I think that providing the functionality here is worth the tradeoff, but I understand any hesitation to add yet more to Ruby!

---Files--------------------------------
scan_left_example.rb (2.93 KB)


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