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feat: Add unfold command (#10489)

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# Description
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Thank you for improving Nushell. Please, check our [contributing
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Description of your pull request goes here. **Provide examples and/or
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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

> **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
> ```
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# After Submitting
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documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.
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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.
This commit is contained in:
Hudson Clark 2023-09-30 10:08:06 -04:00 committed by GitHub
parent 7eaa6d01ab
commit fa2e6e5d53
No known key found for this signature in database
GPG key ID: 4AEE18F83AFDEB23
6 changed files with 341 additions and 3 deletions
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1
crates/nu-command/src/default_context.rs
View file

@ -382,6 +382,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
Seq,
SeqDate,
SeqChar,
Unfold,
};
// Hash

7
crates/nu-command/src/example_test.rs
View file

@ -9,9 +9,9 @@ pub fn test_examples(cmd: impl Command + 'static) {
#[cfg(test)]
mod test_examples {
use super::super::{
Ansi, Date, Enumerate, Filter, Flatten, From, Get, Into, IntoDatetime, IntoString, Math,
MathRound, ParEach, Path, PathParse, Random, Sort, SortBy, Split, SplitColumn, SplitRow,
Str, StrJoin, StrLength, StrReplace, Update, Url, Values, Wrap,
Ansi, Date, Enumerate, Filter, First, Flatten, From, Get, Into, IntoDatetime, IntoString,
Math, MathRound, ParEach, Path, PathParse, Random, Sort, SortBy, Split, SplitColumn,
SplitRow, Str, StrJoin, StrLength, StrReplace, Update, Url, Values, Wrap,
};
use crate::{Each, To};
use nu_cmd_lang::example_support::{
@ -71,6 +71,7 @@ mod test_examples {
working_set.add_decl(Box::new(Echo));
working_set.add_decl(Box::new(Enumerate));
working_set.add_decl(Box::new(Filter));
working_set.add_decl(Box::new(First));
working_set.add_decl(Box::new(Flatten));
working_set.add_decl(Box::new(From));
working_set.add_decl(Box::new(Get));

2
crates/nu-command/src/generators/mod.rs
View file

@ -2,8 +2,10 @@ mod cal;
mod seq;
mod seq_char;
mod seq_date;
mod unfold;
pub use cal::Cal;
pub use seq::Seq;
pub use seq_char::SeqChar;
pub use seq_date::SeqDate;
pub use unfold::Unfold;

231
crates/nu-command/src/generators/unfold.rs Normal file
View file

@ -0,0 +1,231 @@
use itertools::unfold;
use nu_engine::{eval_block_with_early_return, CallExt};
use nu_protocol::ast::Call;
use nu_protocol::engine::{Closure, Command, EngineState, Stack};
use nu_protocol::{
Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, ShellError,
Signature, Span, Spanned, SyntaxShape, Type, Value,
};
#[derive(Clone)]
pub struct Unfold;
impl Command for Unfold {
fn name(&self) -> &str {
"unfold"
}
fn signature(&self) -> Signature {
Signature::build("unfold")
.input_output_types(vec![
(Type::Nothing, Type::List(Box::new(Type::Any))),
(
Type::List(Box::new(Type::Any)),
Type::List(Box::new(Type::Any)),
),
])
.required("initial", SyntaxShape::Any, "initial value")
.required(
"closure",
SyntaxShape::Closure(Some(vec![SyntaxShape::Any])),
"generator function",
)
.allow_variants_without_examples(true)
.category(Category::Generators)
}
fn usage(&self) -> &str {
"Generate a list of values by successively invoking a closure."
}
fn extra_usage(&self) -> &str {
r#"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.
"#
}
fn search_terms(&self) -> Vec<&str> {
vec!["generate", "stream"]
}
fn examples(&self) -> Vec<Example> {
vec![
Example {
example: "unfold 0 {|i| if $i <= 10 { {out: $i, next: ($i + 2)} }}",
description: "Generate a sequence of numbers",
result: Some(Value::list(
vec![
Value::test_int(0),
Value::test_int(2),
Value::test_int(4),
Value::test_int(6),
Value::test_int(8),
Value::test_int(10),
],
Span::test_data(),
)),
},
Example {
example: "unfold [0, 1] {|fib| {out: $fib.0, next: [$fib.1, ($fib.0 + $fib.1)]} } | first 10",
description: "Generate a stream of fibonacci numbers",
result: Some(Value::list(
vec![
Value::test_int(0),
Value::test_int(1),
Value::test_int(1),
Value::test_int(2),
Value::test_int(3),
Value::test_int(5),
Value::test_int(8),
Value::test_int(13),
Value::test_int(21),
Value::test_int(34),
],
Span::test_data(),
)),
},
]
}
fn run(
&self,
engine_state: &EngineState,
stack: &mut Stack,
call: &Call,
_input: PipelineData,
) -> Result<PipelineData, ShellError> {
let initial: Value = call.req(engine_state, stack, 0)?;
let capture_block: Spanned<Closure> = call.req(engine_state, stack, 1)?;
let block_span = capture_block.span;
let block = engine_state.get_block(capture_block.item.block_id).clone();
let ctrlc = engine_state.ctrlc.clone();
let engine_state = engine_state.clone();
let mut stack = stack.captures_to_stack(&capture_block.item.captures);
let orig_env_vars = stack.env_vars.clone();
let orig_env_hidden = stack.env_hidden.clone();
let redirect_stdout = call.redirect_stdout;
let redirect_stderr = call.redirect_stderr;
// A type of Option<S> is used to represent state. Invocation
// will stop on None. Using Option<S> allows functions to output
// one final value before stopping.
let iter = unfold(Some(initial), move |state| {
let arg = match state {
Some(state) => state.clone(),
None => return None,
};
// with_env() is used here to ensure that each iteration uses
// a different set of environment variables.
// Hence, a 'cd' in the first loop won't affect the next loop.
stack.with_env(&orig_env_vars, &orig_env_hidden);
if let Some(var) = block.signature.get_positional(0) {
if let Some(var_id) = &var.var_id {
stack.add_var(*var_id, arg.clone());
}
}
let (output, next_input) = match eval_block_with_early_return(
&engine_state,
&mut stack,
&block,
arg.into_pipeline_data(),
redirect_stdout,
redirect_stderr,
) {
// no data -> output nothing and stop.
Ok(PipelineData::Empty) => (None, None),
Ok(PipelineData::Value(value, ..)) => {
let span = value.span();
match value {
// {out: ..., next: ...} -> output and continue
Value::Record { val, .. } => {
let iter = val.into_iter();
let mut out = None;
let mut next = None;
let mut err = None;
for (k, v) in iter {
if k.to_lowercase() == "out" {
out = Some(v);
} else if k.to_lowercase() == "next" {
next = Some(v);
} else {
let error = ShellError::GenericError(
"Invalid block return".to_string(),
format!("Unexpected record key '{}'", k),
Some(span),
None,
Vec::new(),
);
err = Some(Value::error(error, block_span));
break;
}
}
if err.is_some() {
(err, None)
} else {
(out, next)
}
}
// some other value -> error and stop
_ => {
let error = ShellError::GenericError(
"Invalid block return".to_string(),
format!("Expected record, found {}", value.get_type()),
Some(span),
None,
Vec::new(),
);
(Some(Value::error(error, block_span)), None)
}
}
}
Ok(other) => {
let val = other.into_value(block_span);
let error = ShellError::GenericError(
"Invalid block return".to_string(),
format!("Expected record, found {}", val.get_type()),
Some(val.span()),
None,
Vec::new(),
);
(Some(Value::error(error, block_span)), None)
}
// error -> error and stop
Err(error) => (Some(Value::error(error, block_span)), None),
};
// We use `state` to control when to stop, not `output`. By wrapping
// it in a `Some`, we allow the generator to output `None` as a valid output
// value.
*state = next_input;
Some(output)
});
Ok(iter.flatten().into_pipeline_data(ctrlc))
}
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn test_examples() {
use crate::test_examples;
test_examples(Unfold {})
}
}

1
crates/nu-command/tests/commands/mod.rs
View file

@ -103,6 +103,7 @@ mod touch;
mod transpose;
mod try_;
mod ucp;
mod unfold;
mod uniq;
mod uniq_by;
mod update;

102
crates/nu-command/tests/commands/unfold.rs Normal file
View file

@ -0,0 +1,102 @@
use nu_test_support::{nu, pipeline};
#[test]
fn unfold_no_next_break() {
let actual =
nu!("unfold 1 {|x| if $x == 3 { {out: $x}} else { {out: $x, next: ($x + 1)} }} | to nuon");
assert_eq!(actual.out, "[1, 2, 3]");
}
#[test]
fn unfold_null_break() {
let actual = nu!("unfold 1 {|x| if $x <= 3 { {out: $x, next: ($x + 1)} }} | to nuon");
assert_eq!(actual.out, "[1, 2, 3]");
}
#[test]
fn unfold_allows_empty_output() {
let actual = nu!(pipeline(
r#"
unfold 0 {|x|
if $x == 1 {
{next: ($x + 1)}
} else if $x < 3 {
{out: $x, next: ($x + 1)}
}
} | to nuon
"#
));
assert_eq!(actual.out, "[0, 2]");
}
#[test]
fn unfold_allows_no_output() {
let actual = nu!(pipeline(
r#"
unfold 0 {|x|
if $x < 3 {
{next: ($x + 1)}
}
} | to nuon
"#
));
assert_eq!(actual.out, "[]");
}
#[test]
fn unfold_allows_null_state() {
let actual = nu!(pipeline(
r#"
unfold 0 {|x|
if $x == null {
{out: "done"}
} else if $x < 1 {
{out: "going", next: ($x + 1)}
} else {
{out: "stopping", next: null}
}
} | to nuon
"#
));
assert_eq!(actual.out, "[going, stopping, done]");
}
#[test]
fn unfold_allows_null_output() {
let actual = nu!(pipeline(
r#"
unfold 0 {|x|
if $x == 3 {
{out: "done"}
} else {
{out: null, next: ($x + 1)}
}
} | to nuon
"#
));
assert_eq!(actual.out, "[null, null, null, done]");
}
#[test]
fn unfold_disallows_extra_keys() {
let actual = nu!("unfold 0 {|x| {foo: bar, out: $x}}");
assert!(actual.err.contains("Invalid block return"));
}
#[test]
fn unfold_disallows_list() {
let actual = nu!("unfold 0 {|x| [$x, ($x + 1)]}");
assert!(actual.err.contains("Invalid block return"));
}
#[test]
fn unfold_disallows_primitive() {
let actual = nu!("unfold 0 {|x| 1}");
assert!(actual.err.contains("Invalid block return"));
}