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# Description
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> [!NOTE]
> This PR description originally used examples where the `generator`
closure returned a list. It has since been updated to use records
instead.
The `unfold` command allows users to dynamically generate streams of
data. The stream is generated by repeatedly invoking a `generator`
closure. The `generator` closure accepts a single argument and returns a
record containing two optional keys: 'out' and 'next'. Each invocation,
the 'out' value, if present, is added to the stream. If a 'next' key is
present, it is used as the next argument to the closure, otherwise
generation stops.
The name "unfold" is borrowed from other functional-programming
languages. Whereas `fold` (or `reduce`) takes a stream of values and
outputs a single value, `unfold` takes a single value and outputs a
stream of values.
### Examples
A common example of using `unfold` is to generate a fibbonacci sequence.
See
[here](6ffdac103c/src/sources.rs (L65))
for an example of this in rust's `itertools`.
```nushell
> unfold [0, 1] {|fib| {out: $fib.0, next: [$fib.1, ($fib.0 + $fib.1)]} } | first 10
───┬────
0 │ 0
1 │ 1
2 │ 1
3 │ 2
4 │ 3
5 │ 5
6 │ 8
7 │ 13
8 │ 21
9 │ 34
───┴────
```
This command is particularly useful when consuming paginated APIs, like
Github's. Previously, nushell users might use a loop and buffer
responses into a list, before returning all responses at once. However,
this behavior is not desirable if the result result is very large. Using
`unfold` avoids buffering and allows subsequent pipeline stages to use
the data concurrently, as it's being fetched.
#### Before
```nushell
mut pages = []
for page in 1.. {
let resp = http get (
{
scheme: https,
host: "api.github.com",
path: "/repos/nushell/nushell/issues",
params: {
page: $page,
per_page: $PAGE_SIZE
}
} | url join)
$pages = ($pages | append $resp)
if ($resp | length) < $PAGE_SIZE {
break
}
}
$pages
```
#### After
```nu
unfold 1 {|page|
let resp = http get (
{
scheme: https,
host: "api.github.com",
path: "/repos/nushell/nushell/issues",
params: {
page: $page,
per_page: $PAGE_SIZE
}
} | url join)
if ($resp | length) < $PAGE_SIZE {
{out: $resp}
} else {
{out: $resp, next: ($page + 1)}
}
}
```
# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
- An `unfold` generator is added to the default context.
# Tests + Formatting
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sure to [enable developer
mode](https://learn.microsoft.com/en-us/windows/apps/get-started/developer-mode-features-and-debugging))
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crates/nu-std"` to run the tests for the standard library
> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->
# After Submitting
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documentation](https://github.com/nushell/nushell.github.io) after the
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Given the complexity of the `generator` closure's return value, it would
be good to document the semantics of `unfold` and provide some in-depth
examples showcasing what it can accomplish.
Hudson Clark