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use nu_engine::command_prelude::*;
use nu_protocol::{Signals, shell_error::io::IoError};
use std::io::Read;
#[cfg(feature = "sqlite")]
use crate::database::SQLiteQueryBuilder;
#[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 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.",
)
.switch("strict", "Throw an error if input is empty.", Some('s'))
.allow_variants_without_examples(true)
.category(Category::Filters)
}
fn search_terms(&self) -> Vec<&str> {
vec!["head"]
}
fn description(&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())),
},
Example {
description: "Return the first item of a range.",
example: "1..3 | first",
result: Some(Value::test_int(1)),
},
]
}
}
fn first_helper(
engine_state: &EngineState,
stack: &mut Stack,
call: &Call,
input: PipelineData,
) -> Result<PipelineData, ShellError> {
let head = call.head;
let rows: Option<Spanned<i64>> = call.opt(engine_state, stack, 0)?;
let strict_mode = call.has_flag(engine_state, stack, "strict")?;
// 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 = if let Some(rows) = rows {
if rows.item < 0 {
return Err(ShellError::NeedsPositiveValue { span: rows.span });
} else {
rows.item as usize
}
} else {
1
};
let mut input = input;
let input_meta = input.take_metadata();
// Count is 0: return empty data immediately.
//
// The main `match` below is not safe for this case-byte streams can still be read from the
// pipe, and sqlite lazy queries can still run. For "take nothing" we only produce an empty
// value: empty binary (and clear pipeline `content_type` for binary) or an empty list, with
// other metadata unchanged.
if rows == 0 {
return match input {
PipelineData::Value(val, _) if matches!(&val, Value::Binary { .. }) => Ok(
Value::binary(Vec::new(), val.span()).into_pipeline_data_with_metadata(
input_meta.map(|m| m.with_content_type(None)),
),
),
PipelineData::ByteStream(stream, _) => {
if stream.type_().is_binary_coercible() {
let span = stream.span();
Ok(
Value::binary(Vec::new(), span).into_pipeline_data_with_metadata(
input_meta.map(|m| m.with_content_type(None)),
),
)
} else {
Ok(Value::list(Vec::new(), head).into_pipeline_data_with_metadata(input_meta))
}
}
_ => Ok(Value::list(Vec::new(), head).into_pipeline_data_with_metadata(input_meta)),
};
}
match input {
PipelineData::Value(val, _) => {
let span = val.span();
match val {
Value::List { mut vals, .. } => {
if return_single_element {
if let Some(val) = vals.first_mut() {
Ok(std::mem::take(val).into_pipeline_data_with_metadata(input_meta))
} else if strict_mode {
Err(ShellError::AccessEmptyContent { span: head })
} else {
// There are no values, so return nothing instead of an error so
// that users can pipe this through 'default' if they want to.
Ok(Value::nothing(head).into_pipeline_data_with_metadata(input_meta))
}
} else {
vals.truncate(rows);
Ok(Value::list(vals, span).into_pipeline_data_with_metadata(input_meta))
}
}
Value::Binary { mut val, .. } => {
// A slice (or single byte as int) is not the whole file/stream; drop MIME.
let binary_meta = input_meta.map(|m| m.with_content_type(None));
if return_single_element {
if let Some(&val) = val.first() {
Ok(Value::int(val.into(), span)
.into_pipeline_data_with_metadata(binary_meta))
} else if strict_mode {
Err(ShellError::AccessEmptyContent { span: head })
} else {
// There are no values, so return nothing instead of an error so
// that users can pipe this through 'default' if they want to.
Ok(Value::nothing(head).into_pipeline_data_with_metadata(binary_meta))
}
} else {
val.truncate(rows);
Ok(Value::binary(val, span).into_pipeline_data_with_metadata(binary_meta))
}
}
Value::Range { val, .. } => {
let mut iter = val.into_range_iter(span, Signals::empty());
if return_single_element {
if let Some(v) = iter.next() {
Ok(v.into_pipeline_data_with_metadata(input_meta))
} else if strict_mode {
Err(ShellError::AccessEmptyContent { span: head })
} else {
// There are no values, so return nothing instead of an error so
// that users can pipe this through 'default' if they want to.
Ok(Value::nothing(head).into_pipeline_data_with_metadata(input_meta))
}
} else {
Ok(iter.take(rows).into_pipeline_data_with_metadata(
span,
engine_state.signals().clone(),
input_meta,
))
}
}
// Propagate errors by explicitly matching them before the final case.
Value::Error { error, .. } => Err(*error),
#[cfg(feature = "sqlite")]
// Pushdown optimization: handle 'first' on SQLiteQueryBuilder for lazy SQL execution
Value::Custom {
val: custom_val,
internal_span,
..
} => {
if let Some(table) = custom_val.as_any().downcast_ref::<SQLiteQueryBuilder>() {
if return_single_element {
// For single element, limit 1
let new_table = table.clone().with_limit(1);
let result = new_table.execute(head)?;
let value = result.into_value(head)?;
if let Value::List { vals, .. } = value {
if let Some(val) = vals.into_iter().next() {
Ok(val.into_pipeline_data_with_metadata(input_meta))
} else if strict_mode {
Err(ShellError::AccessEmptyContent { span: head })
} else {
// There are no values, so return nothing instead of an error so
// that users can pipe this through 'default' if they want to.
Ok(Value::nothing(head)
.into_pipeline_data_with_metadata(input_meta))
}
} else {
Err(ShellError::NushellFailed {
msg: "Expected list from SQLiteQueryBuilder".into(),
})
}
} else {
// For multiple, limit rows
let new_table = table.clone().with_limit(rows as i64);
new_table
.execute(head)
.map(|data| data.set_metadata(input_meta))
}
} else {
Err(ShellError::OnlySupportsThisInputType {
exp_input_type: "list, binary or range".into(),
wrong_type: custom_val.type_name(),
dst_span: head,
src_span: internal_span,
})
}
}
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(stream, _) => {
if return_single_element {
if let Some(v) = stream.into_iter().next() {
Ok(v.into_pipeline_data_with_metadata(input_meta))
} else if strict_mode {
Err(ShellError::AccessEmptyContent { span: head })
} else {
// There are no values, so return nothing instead of an error so
// that users can pipe this through 'default' if they want to.
Ok(Value::nothing(head).into_pipeline_data_with_metadata(input_meta))
}
} else {
Ok(PipelineData::list_stream(
stream.modify(|iter| iter.take(rows)),
input_meta,
))
}
}
PipelineData::ByteStream(stream, _) => {
if stream.type_().is_binary_coercible() {
let span = stream.span();
let metadata = input_meta.map(|m| m.with_content_type(None));
if let Some(mut reader) = stream.reader() {
if return_single_element {
// Take a single byte
let mut byte = [0u8];
if reader
.read(&mut byte)
.map_err(|err| IoError::new(err, span, None))?
> 0
{
Ok(Value::int(byte[0] as i64, head)
.into_pipeline_data_with_metadata(metadata))
} else {
Err(ShellError::AccessEmptyContent { span: head })
}
} else {
// Just take 'rows' bytes off the stream, mimicking the binary behavior
Ok(PipelineData::byte_stream(
ByteStream::read(
reader.take(rows as u64),
head,
Signals::empty(),
ByteStreamType::Binary,
),
metadata,
))
}
} else {
Ok(Value::nothing(head).into_pipeline_data_with_metadata(metadata))
}
} else {
Err(ShellError::OnlySupportsThisInputType {
exp_input_type: "list, binary or range".into(),
wrong_type: stream.type_().describe().into(),
dst_span: head,
src_span: stream.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() -> nu_test_support::Result {
nu_test_support::test().examples(First)
}
}