Port skip, skip while and skip until commands (#380)

* Add `Skip` command

* Add `SkipUntil` sub-command

* Add `SkipWhile` sub-command

* Add and use `Expression::as_row_condition_block`

crates/nu-command/src/default_context.rs

@@ -116,6 +116,9 @@ pub fn create_default_context() -> EngineState {
             Select,
             Shuffle,
             Size,
+            Skip,
+            SkipUntil,
+            SkipWhile,
             Sleep,
             Source,
             Split,

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

@@ -1,6 +1,6 @@
 use nu_engine::eval_block;
 use nu_protocol::{
-    ast::{Call, Expr, Expression},
+    ast::Call,
     engine::{Command, EngineState, Stack},
     Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, SyntaxShape,
 };
@@ -52,13 +52,9 @@ impl Command for All {
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
         let predicate = &call.positional[0];
-        let block_id = match predicate {
-            Expression {
-                expr: Expr::RowCondition(block_id),
-                ..
-            } => *block_id,
-            _ => return Err(ShellError::InternalError("Expected row condition".into())),
-        };
+        let block_id = predicate
+            .as_row_condition_block()
+            .ok_or_else(|| ShellError::InternalError("Expected row condition".to_owned()))?;
 
         let span = call.head;
 

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

@@ -1,6 +1,6 @@
 use nu_engine::eval_block;
 use nu_protocol::{
-    ast::{Call, Expr, Expression},
+    ast::Call,
     engine::{Command, EngineState, Stack},
     Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, SyntaxShape,
 };
@@ -52,13 +52,9 @@ impl Command for Any {
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
         let predicate = &call.positional[0];
-        let block_id = match predicate {
-            Expression {
-                expr: Expr::RowCondition(block_id),
-                ..
-            } => *block_id,
-            _ => return Err(ShellError::InternalError("Expected row condition".into())),
-        };
+        let block_id = predicate
+            .as_row_condition_block()
+            .ok_or_else(|| ShellError::InternalError("Expected row condition".to_owned()))?;
 
         let span = call.head;
 

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

@@ -14,6 +14,7 @@ mod range;
 mod reverse;
 mod select;
 mod shuffle;
+mod skip;
 mod update;
 mod where_;
 mod wrap;
@@ -35,6 +36,7 @@ pub use range::Range;
 pub use reverse::Reverse;
 pub use select::Select;
 pub use shuffle::Shuffle;
+pub use skip::*;
 pub use update::Update;
 pub use where_::Where;
 pub use wrap::Wrap;

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

@@ -0,0 +1,90 @@
+use std::convert::TryInto;
+
+use nu_engine::CallExt;
+use nu_protocol::{
+    ast::Call,
+    engine::{Command, EngineState, Stack},
+    Category, Example, IntoInterruptiblePipelineData, PipelineData, ShellError, Signature, Span,
+    SyntaxShape, Value,
+};
+
+#[derive(Clone)]
+pub struct Skip;
+
+impl Command for Skip {
+    fn name(&self) -> &str {
+        "skip"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .optional("n", SyntaxShape::Int, "the number of elements to skip")
+            .category(Category::Filters)
+    }
+
+    fn usage(&self) -> &str {
+        "Skip the first n elements of the input."
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Skip two elements",
+                example: "echo [[editions]; [2015] [2018] [2021]] | skip 2",
+                result: Some(Value::List {
+                    vals: vec![Value::Record {
+                        cols: vec!["editions".to_owned()],
+                        vals: vec![Value::from(2021)],
+                        span: Span::unknown(),
+                    }],
+                    span: Span::unknown(),
+                }),
+            },
+            Example {
+                description: "Skip the first value",
+                example: "echo [2 4 6 8] | skip",
+                result: Some(Value::List {
+                    vals: vec![Value::from(4), Value::from(6), Value::from(8)],
+                    span: Span::unknown(),
+                }),
+            },
+        ]
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let n: Option<Value> = call.opt(engine_state, stack, 0)?;
+
+        let n: usize = match n {
+            Some(Value::Int { val, span }) => val.try_into().map_err(|err| {
+                ShellError::UnsupportedInput(
+                    format!("Could not convert {} to unsigned integer: {}", val, err),
+                    span,
+                )
+            })?,
+            Some(_) => return Err(ShellError::InternalError("Expected integer".into())),
+            None => 1,
+        };
+
+        let ctrlc = engine_state.ctrlc.clone();
+
+        Ok(input.into_iter().skip(n).into_pipeline_data(ctrlc))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(Skip {})
+    }
+}

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

@@ -0,0 +1,7 @@
+mod command;
+mod until;
+mod while_;
+
+pub use command::Skip;
+pub use until::SkipUntil;
+pub use while_::SkipWhile;

crates/nu-command/src/filters/skip/until.rs

@@ -0,0 +1,89 @@
+use nu_engine::eval_block;
+use nu_protocol::{
+    ast::Call,
+    engine::{Command, EngineState, Stack},
+    Category, Example, IntoInterruptiblePipelineData, PipelineData, ShellError, Signature, Span,
+    SyntaxShape, Value,
+};
+
+#[derive(Clone)]
+pub struct SkipUntil;
+
+impl Command for SkipUntil {
+    fn name(&self) -> &str {
+        "skip until"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .required(
+                "predicate",
+                SyntaxShape::RowCondition,
+                "the predicate that skipped element must not match",
+            )
+            .category(Category::Filters)
+    }
+
+    fn usage(&self) -> &str {
+        "Skip elements of the input until a predicate is true."
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Skip until the element is positive",
+            example: "echo [-2 0 2 -1] | skip until $it > 0",
+            result: Some(Value::List {
+                vals: vec![Value::from(2), Value::from(-1)],
+                span: Span::unknown(),
+            }),
+        }]
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let predicate = &call.positional[0];
+        let block_id = predicate
+            .as_row_condition_block()
+            .ok_or_else(|| ShellError::InternalError("Expected row condition".to_owned()))?;
+
+        let span = call.head;
+
+        let block = engine_state.get_block(block_id).clone();
+        let var_id = block.signature.get_positional(0).and_then(|arg| arg.var_id);
+        let mut stack = stack.collect_captures(&block.captures);
+
+        let ctrlc = engine_state.ctrlc.clone();
+        let engine_state = engine_state.clone();
+
+        Ok(input
+            .into_iter()
+            .skip_while(move |value| {
+                if let Some(var_id) = var_id {
+                    stack.add_var(var_id, value.clone());
+                }
+
+                !eval_block(&engine_state, &mut stack, &block, PipelineData::new(span))
+                    .map_or(false, |pipeline_data| {
+                        pipeline_data.into_value(span).is_true()
+                    })
+            })
+            .into_pipeline_data(ctrlc))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(SkipUntil)
+    }
+}

crates/nu-command/src/filters/skip/while_.rs

@@ -0,0 +1,89 @@
+use nu_engine::eval_block;
+use nu_protocol::{
+    ast::Call,
+    engine::{Command, EngineState, Stack},
+    Category, Example, IntoInterruptiblePipelineData, PipelineData, ShellError, Signature, Span,
+    SyntaxShape, Value,
+};
+
+#[derive(Clone)]
+pub struct SkipWhile;
+
+impl Command for SkipWhile {
+    fn name(&self) -> &str {
+        "skip while"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .required(
+                "predicate",
+                SyntaxShape::RowCondition,
+                "the predicate that skipped element must match",
+            )
+            .category(Category::Filters)
+    }
+
+    fn usage(&self) -> &str {
+        "Skip elements of the input while a predicate is true."
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Skip while the element is negative",
+            example: "echo [-2 0 2 -1] | skip while $it < 0",
+            result: Some(Value::List {
+                vals: vec![Value::from(0), Value::from(2), Value::from(-1)],
+                span: Span::unknown(),
+            }),
+        }]
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let predicate = &call.positional[0];
+        let block_id = predicate
+            .as_row_condition_block()
+            .ok_or_else(|| ShellError::InternalError("Expected row condition".to_owned()))?;
+
+        let span = call.head;
+
+        let block = engine_state.get_block(block_id).clone();
+        let var_id = block.signature.get_positional(0).and_then(|arg| arg.var_id);
+        let mut stack = stack.collect_captures(&block.captures);
+
+        let ctrlc = engine_state.ctrlc.clone();
+        let engine_state = engine_state.clone();
+
+        Ok(input
+            .into_iter()
+            .skip_while(move |value| {
+                if let Some(var_id) = var_id {
+                    stack.add_var(var_id, value.clone());
+                }
+
+                eval_block(&engine_state, &mut stack, &block, PipelineData::new(span))
+                    .map_or(false, |pipeline_data| {
+                        pipeline_data.into_value(span).is_true()
+                    })
+            })
+            .into_pipeline_data(ctrlc))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(SkipWhile)
+    }
+}

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

@@ -1,5 +1,5 @@
 use nu_engine::eval_block;
-use nu_protocol::ast::{Call, Expr, Expression};
+use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{Category, PipelineData, ShellError, Signature, SyntaxShape};
 
@@ -29,19 +29,14 @@ impl Command for Where {
         input: PipelineData,
     ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
         let span = call.head;
-        let cond = call.positional[0].clone();
+        let cond = &call.positional[0];
+        let block_id = cond
+            .as_row_condition_block()
+            .ok_or_else(|| ShellError::InternalError("Expected row condition".to_owned()))?;
 
         let ctrlc = engine_state.ctrlc.clone();
         let engine_state = engine_state.clone();
 
-        let block_id = match cond {
-            Expression {
-                expr: Expr::RowCondition(block_id),
-                ..
-            } => block_id,
-            _ => return Err(ShellError::InternalError("Expected row condition".into())),
-        };
-
         let block = engine_state.get_block(block_id).clone();
         let mut stack = stack.collect_captures(&block.captures);
 

crates/nu-protocol/src/ast/expression.rs

@@ -53,6 +53,13 @@ impl Expression {
         }
     }
 
+    pub fn as_row_condition_block(&self) -> Option<BlockId> {
+        match self.expr {
+            Expr::RowCondition(block_id) => Some(block_id),
+            _ => None,
+        }
+    }
+
     pub fn as_signature(&self) -> Option<Box<Signature>> {
         match &self.expr {
             Expr::Signature(sig) => Some(sig.clone()),