Merge pull request #277 from onthebridgetonowhere/port_first_command

Port first command

crates/nu-command/src/default_context.rs

@@ -40,6 +40,7 @@ pub fn create_default_context() -> EngineState {
             Echo,
             ExportDef,
             External,
+            First,
             For,
             From,
             FromJson,

crates/nu-command/src/filters/first.rs

@@ -0,0 +1,94 @@
+use nu_engine::CallExt;
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{
+    Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+};
+
+#[derive(Clone)]
+pub struct First;
+
+impl Command for First {
+    fn name(&self) -> &str {
+        "first"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("first").optional(
+            "rows",
+            SyntaxShape::Int,
+            "starting from the front, the number of rows to return",
+        )
+    }
+
+    fn usage(&self) -> &str {
+        "Show only the first number of rows."
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<nu_protocol::PipelineData, nu_protocol::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 {
+                    vals: vec![Value::test_int(1), Value::test_int(2)],
+                    span: Span::unknown(),
+                }),
+            },
+        ]
+    }
+}
+
+fn first_helper(
+    engine_state: &EngineState,
+    stack: &mut Stack,
+    call: &Call,
+    input: PipelineData,
+) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
+    let head = call.head;
+    let rows: Option<i64> = call.opt(engine_state, stack, 0)?;
+    let rows_desired: usize = match rows {
+        Some(x) => x as usize,
+        None => 1,
+    };
+
+    if rows_desired == 1 {
+        let mut input_peek = input.into_iter().peekable();
+        match input_peek.next() {
+            Some(val) => Ok(val.into_pipeline_data()),
+            None => Err(ShellError::AccessBeyondEndOfStream(head)),
+        }
+    } else {
+        Ok(Value::List {
+            vals: input.into_iter().take(rows_desired).collect(),
+            span: head,
+        }
+        .into_pipeline_data())
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(First {})
+    }
+}

crates/nu-command/src/filters/mod.rs

@@ -1,4 +1,5 @@
 mod each;
+mod first;
 mod get;
 mod last;
 mod length;
@@ -11,6 +12,7 @@ mod wrap;
 mod zip;
 
 pub use each::Each;
+pub use first::First;
 pub use get::Get;
 pub use last::Last;
 pub use length::Length;