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first.rs
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first.rs
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use nu_engine::CallExt;
use nu_protocol::ast::Call;
use nu_protocol::engine::{Command, EngineState, Stack};
use nu_protocol::{
Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, ShellError,
Signature, Span, SyntaxShape, Type, Value,
};
#[derive(Clone)]
pub struct First;
impl Command for First {
fn name(&self) -> &str {
"first"
}
fn signature(&self) -> Signature {
Signature::build("first")
.input_output_types(vec![
(
// TODO: This variant duplicates the functionality of
// `take`. See #6611, #6611, #6893
// TODO: This is too permissive; if we could express this
// using a type parameter style it would be List<T> ->
// List<T>.
Type::List(Box::new(Type::Any)),
Type::List(Box::new(Type::Any)),
),
(
// TODO: This is too permissive; if we could express this
// using a type parameter it would be List<T> -> T.
Type::List(Box::new(Type::Any)),
Type::Any,
),
(Type::Binary, Type::Binary),
(Type::Range, Type::Any),
])
.optional(
"rows",
SyntaxShape::Int,
"starting from the front, the number of rows to return",
)
.allow_variants_without_examples(true)
.category(Category::Filters)
}
fn usage(&self) -> &str {
"Return only the first several rows of the input. Counterpart of `last`. Opposite of `skip`."
}
fn run(
&self,
engine_state: &EngineState,
stack: &mut Stack,
call: &Call,
input: PipelineData,
) -> Result<PipelineData, ShellError> {
first_helper(engine_state, stack, call, input)
}
fn examples(&self) -> Vec<Example> {
vec![
Example {
description: "Return the first item of a list/table",
example: "[1 2 3] | first",
result: Some(Value::test_int(1)),
},
Example {
description: "Return the first 2 items of a list/table",
example: "[1 2 3] | first 2",
result: Some(Value::list(
vec![Value::test_int(1), Value::test_int(2)],
Span::test_data(),
)),
},
Example {
description: "Return the first 2 bytes of a binary value",
example: "0x[01 23 45] | first 2",
result: Some(Value::binary(vec![0x01, 0x23], Span::test_data())),
},
]
}
}
fn first_helper(
engine_state: &EngineState,
stack: &mut Stack,
call: &Call,
input: PipelineData,
) -> Result<PipelineData, ShellError> {
let head = call.head;
let rows: Option<i64> = call.opt(engine_state, stack, 0)?;
// FIXME: for backwards compatibility reasons, if `rows` is not specified we
// return a single element and otherwise we return a single list. We should probably
// remove `rows` so that `first` always returns a single element; getting a list of
// the first N elements is covered by `take`
let return_single_element = rows.is_none();
let rows_desired: usize = match rows {
Some(i) if i < 0 => return Err(ShellError::NeedsPositiveValue(head)),
Some(x) => x as usize,
None => 1,
};
let ctrlc = engine_state.ctrlc.clone();
let metadata = input.metadata();
// early exit for `first 0`
if rows_desired == 0 {
return Ok(Vec::<Value>::new().into_pipeline_data_with_metadata(metadata, ctrlc));
}
match input {
PipelineData::Value(val, _) => {
let span = val.span();
match val {
Value::List { vals, .. } => {
if return_single_element {
if vals.is_empty() {
Err(ShellError::AccessEmptyContent { span: head })
} else {
Ok(vals[0].clone().into_pipeline_data())
}
} else {
Ok(vals
.into_iter()
.take(rows_desired)
.into_pipeline_data_with_metadata(metadata, ctrlc))
}
}
Value::Binary { val, .. } => {
if return_single_element {
if val.is_empty() {
Err(ShellError::AccessEmptyContent { span: head })
} else {
Ok(PipelineData::Value(
Value::int(val[0] as i64, span),
metadata,
))
}
} else {
let slice: Vec<u8> = val.into_iter().take(rows_desired).collect();
Ok(PipelineData::Value(Value::binary(slice, span), metadata))
}
}
Value::Range { val, .. } => {
if return_single_element {
Ok(val.from.into_pipeline_data())
} else {
Ok(val
.into_range_iter(ctrlc.clone())?
.take(rows_desired)
.into_pipeline_data_with_metadata(metadata, ctrlc))
}
}
// Propagate errors by explicitly matching them before the final case.
Value::Error { error, .. } => Err(*error),
other => Err(ShellError::OnlySupportsThisInputType {
exp_input_type: "list, binary or range".into(),
wrong_type: other.get_type().to_string(),
dst_span: head,
src_span: other.span(),
}),
}
}
PipelineData::ListStream(mut ls, metadata) => {
if return_single_element {
if let Some(v) = ls.next() {
Ok(v.into_pipeline_data())
} else {
Err(ShellError::AccessEmptyContent { span: head })
}
} else {
Ok(ls
.take(rows_desired)
.into_pipeline_data_with_metadata(metadata, ctrlc))
}
}
PipelineData::ExternalStream { span, .. } => Err(ShellError::OnlySupportsThisInputType {
exp_input_type: "list, binary or range".into(),
wrong_type: "raw data".into(),
dst_span: head,
src_span: span,
}),
PipelineData::Empty => Err(ShellError::OnlySupportsThisInputType {
exp_input_type: "list, binary or range".into(),
wrong_type: "null".into(),
dst_span: call.head,
src_span: call.head,
}),
}
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn test_examples() {
use crate::test_examples;
test_examples(First {})
}
}