nushell/crates/nu-command/tests
Hudson Clark fa2e6e5d53
feat: Add unfold command (#10489)
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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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crates/nu-std"` to run the tests for the standard library

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> from `nushell` you can also use the `toolkit` as follows
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# After Submitting
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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.
2023-09-30 09:08:06 -05:00
..
commands feat: Add unfold command (#10489) 2023-09-30 09:08:06 -05:00
format_conversions Remove dead BSON related tests (#10458) 2023-09-21 20:01:49 +02:00
main.rs Rename misused "deprecation" to removal (#10000) 2023-08-15 07:17:31 +12:00