Allow dropping columns. (#3107)

`drop` is used for removing the last row. Passing a number allows dropping N rows.
Here we introduce the same logic for dropping columns instead.

You can certainly remove columns by using `reject`, however, there could be cases
where we are interested in removing columns from tables that contain, say, a big
number of columns. Using `reject` becomes impractical, especially when you don't
care about the column names that could either be known or not known when exploring
tables.

```
> echo [[lib, extension]; [nu-core, rs] [rake, rb]]
─────────┬───────────
   lib   │ extension
─────────┼───────────
 nu-core │ rs
 rake    │ rb
─────────┴───────────
```

```
> echo [[lib, extension]; [nu-core, rs] [rake, rb]] | drop column
─────────
   lib
─────────
 nu-core
 rake
─────────
```

crates/nu-command/src/commands.rs

@@ -163,7 +163,7 @@ pub(crate) use def::Def;
 pub(crate) use default::Default;
 pub(crate) use describe::Describe;
 pub(crate) use do_::Do;
-pub(crate) use drop::Drop;
+pub(crate) use drop::{Drop, DropColumn};
 pub(crate) use du::Du;
 pub(crate) use each::Each;
 pub(crate) use each::EachGroup;

crates/nu-command/src/commands/default_context.rs

@@ -114,6 +114,7 @@ pub fn create_default_context(interactive: bool) -> Result<EvaluationContext, Bo
             whole_stream_command(AnsiStrip),
             whole_stream_command(Char),
             // Column manipulation
+            whole_stream_command(DropColumn),
             whole_stream_command(Move),
             whole_stream_command(Reject),
             whole_stream_command(Select),

crates/nu-command/src/commands/drop/column.rs

@@ -0,0 +1,86 @@
+use crate::prelude::*;
+use nu_data::base::select_fields;
+use nu_engine::WholeStreamCommand;
+use nu_errors::ShellError;
+use nu_protocol::{ReturnSuccess, Signature, SyntaxShape};
+use nu_source::Tagged;
+
+pub struct SubCommand;
+
+#[derive(Deserialize)]
+pub struct Arguments {
+    columns: Option<Tagged<u64>>,
+}
+
+#[async_trait]
+impl WholeStreamCommand for SubCommand {
+    fn name(&self) -> &str {
+        "drop column"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("drop column").optional(
+            "columns",
+            SyntaxShape::Number,
+            "starting from the end, the number of columns to remove",
+        )
+    }
+
+    fn usage(&self) -> &str {
+        "Remove the last number of columns. If you want to remove columns by name, try 'reject'."
+    }
+
+    async fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {
+        drop(args).await
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        use nu_protocol::{row, Value};
+
+        vec![Example {
+            description: "Remove the last column of a table",
+            example: "echo [[lib, extension]; [nu-lib, rs] [nu-core, rb]] | drop column",
+            result: Some(vec![
+                row! { "lib".into() => Value::from("nu-lib") },
+                row! { "lib".into() => Value::from("nu-core") },
+            ]),
+        }]
+    }
+}
+
+async fn drop(args: CommandArgs) -> Result<OutputStream, ShellError> {
+    let (Arguments { columns }, input) = args.process().await?;
+
+    let to_drop = if let Some(quantity) = columns {
+        *quantity as usize
+    } else {
+        1
+    };
+
+    Ok(input
+        .map(move |item| {
+            let headers = item.data_descriptors();
+
+            let descs = match headers.len() {
+                0 => &headers[..],
+                n if to_drop > n => &[],
+                n => &headers[..n - to_drop],
+            };
+
+            select_fields(&item, descs, item.tag())
+        })
+        .map(ReturnSuccess::value)
+        .to_output_stream())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn examples_work_as_expected() -> Result<(), ShellError> {
+        use crate::examples::test as test_examples;
+
+        test_examples(SubCommand {})
+    }
+}

crates/nu-command/src/commands/drop/command.rs

@@ -4,15 +4,15 @@ use nu_errors::ShellError;
 use nu_protocol::{Signature, SyntaxShape, UntaggedValue};
 use nu_source::Tagged;
 
-pub struct Drop;
+pub struct Command;
 
 #[derive(Deserialize)]
-pub struct DropArgs {
+pub struct Arguments {
     rows: Option<Tagged<u64>>,
 }
 
 #[async_trait]
-impl WholeStreamCommand for Drop {
+impl WholeStreamCommand for Command {
     fn name(&self) -> &str {
         "drop"
     }
@@ -26,7 +26,7 @@ impl WholeStreamCommand for Drop {
     }
 
     fn usage(&self) -> &str {
-        "Remove the last number of rows. If you want to remove columns, try 'reject'."
+        "Remove the last number of rows. If you want to remove columns, try 'drop column'."
     }
 
     async fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {
@@ -53,7 +53,7 @@ impl WholeStreamCommand for Drop {
 }
 
 async fn drop(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    let (DropArgs { rows }, input) = args.process().await?;
+    let (Arguments { rows }, input) = args.process().await?;
     let v: Vec<_> = input.into_vec().await;
 
     let rows_to_drop = if let Some(quantity) = rows {
@@ -76,16 +76,3 @@ async fn drop(args: CommandArgs) -> Result<OutputStream, ShellError> {
         futures::stream::iter(iter).to_output_stream()
     })
 }
-
-#[cfg(test)]
-mod tests {
-    use super::Drop;
-    use super::ShellError;
-
-    #[test]
-    fn examples_work_as_expected() -> Result<(), ShellError> {
-        use crate::examples::test as test_examples;
-
-        test_examples(Drop {})
-    }
-}

crates/nu-command/src/commands/drop/mod.rs

@@ -0,0 +1,5 @@
+mod column;
+mod command;
+
+pub use column::SubCommand as DropColumn;
+pub use command::Command as Drop;

crates/nu-command/tests/commands/drop.rs

@@ -1,23 +1,68 @@
 use nu_test_support::{nu, pipeline};
 
 #[test]
-fn drop_rows() {
+fn columns() {
     let actual = nu!(
-        cwd: "tests/fixtures/formats",
-        r#"echo '[{"foo": 3}, {"foo": 8}, {"foo": 4}]' | from json | drop 2 | get foo | math sum "#
+        cwd: ".", pipeline(r#"
+            echo [
+              [arepas, color];
+
+              [3,  white]
+              [8, yellow]
+              [4,  white]
+            ]
+            | drop column
+            | get
+            | count
+        "#)
+    );
+
+    assert_eq!(actual.out, "1");
+}
+
+#[test]
+fn more_columns_than_table_has() {
+    let actual = nu!(
+        cwd: ".", pipeline(r#"
+            echo [
+              [arepas, color];
+
+              [3,  white]
+              [8, yellow]
+              [4,  white]
+            ]
+            | drop column 3
+            | get
+            | empty?
+        "#)
+    );
+
+    assert_eq!(actual.out, "true");
+}
+
+#[test]
+fn rows() {
+    let actual = nu!(
+        cwd: ".", pipeline(r#"
+            echo [
+              [arepas];
+
+              [3]
+              [8]
+              [4]
+            ]
+            | drop 2
+            | get arepas
+            | math sum
+        "#)
     );
 
     assert_eq!(actual.out, "3");
 }
 
 #[test]
-fn drop_more_rows_than_table_has() {
-    let actual = nu!(
-        cwd: ".", pipeline(
-        r#"
-        date | drop 50 | count
-        "#
-    ));
+fn more_rows_than_table_has() {
+    let actual = nu!(cwd: ".", "date | drop 50 | count");
 
     assert_eq!(actual.out, "0");
 }