Add row conditions

crates/nu-command/src/default_context.rs

@@ -5,7 +5,9 @@ use nu_protocol::{
     Signature, SyntaxShape,
 };
 
-use crate::{Alias, Benchmark, BuildString, Def, Do, Each, For, If, Length, Let, LetEnv};
+use crate::{
+    where_::Where, Alias, Benchmark, BuildString, Def, Do, Each, For, If, Length, Let, LetEnv,
+};
 
 pub fn create_default_context() -> Rc<RefCell<EngineState>> {
     let engine_state = Rc::new(RefCell::new(EngineState::new()));
@@ -33,6 +35,8 @@ pub fn create_default_context() -> Rc<RefCell<EngineState>> {
 
         working_set.add_decl(Box::new(Each));
 
+        working_set.add_decl(Box::new(Where));
+
         working_set.add_decl(Box::new(Do));
 
         working_set.add_decl(Box::new(Benchmark));

crates/nu-command/src/if_.rs

@@ -11,7 +11,7 @@ impl Command for If {
     }
 
     fn usage(&self) -> &str {
-        "Create a variable and give it a value."
+        "Conditionally run a block."
     }
 
     fn signature(&self) -> nu_protocol::Signature {

crates/nu-command/src/lib.rs

@@ -10,6 +10,7 @@ mod if_;
 mod length;
 mod let_;
 mod let_env;
+mod where_;
 
 pub use alias::Alias;
 pub use benchmark::Benchmark;

crates/nu-command/src/where_.rs

@@ -0,0 +1,92 @@
+use nu_engine::eval_expression;
+use nu_protocol::ast::{Call, Expr, Expression};
+use nu_protocol::engine::{Command, EvaluationContext};
+use nu_protocol::{IntoValueStream, ShellError, Signature, SyntaxShape, Value};
+
+pub struct Where;
+
+impl Command for Where {
+    fn name(&self) -> &str {
+        "where"
+    }
+
+    fn usage(&self) -> &str {
+        "Filter values based on a condition."
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("where").required("cond", SyntaxShape::RowCondition, "condition")
+    }
+
+    fn run(
+        &self,
+        context: &EvaluationContext,
+        call: &Call,
+        input: Value,
+    ) -> Result<nu_protocol::Value, nu_protocol::ShellError> {
+        let cond = call.positional[0].clone();
+
+        let context = context.enter_scope();
+
+        let (var_id, cond) = match cond {
+            Expression {
+                expr: Expr::RowCondition(var_id, expr),
+                ..
+            } => (var_id, expr),
+            _ => return Err(ShellError::InternalError("Expected row condition".into())),
+        };
+
+        match input {
+            Value::Stream { stream, span } => {
+                let output_stream = stream
+                    .filter(move |value| {
+                        context.add_var(var_id, value.clone());
+
+                        let result = eval_expression(&context, &cond);
+
+                        match result {
+                            Ok(result) => result.is_true(),
+                            _ => false,
+                        }
+                    })
+                    .into_value_stream();
+
+                Ok(Value::Stream {
+                    stream: output_stream,
+                    span,
+                })
+            }
+            Value::List { vals, span } => {
+                let output_stream = vals
+                    .into_iter()
+                    .filter(move |value| {
+                        context.add_var(var_id, value.clone());
+
+                        let result = eval_expression(&context, &cond);
+
+                        match result {
+                            Ok(result) => result.is_true(),
+                            _ => false,
+                        }
+                    })
+                    .into_value_stream();
+
+                Ok(Value::Stream {
+                    stream: output_stream,
+                    span,
+                })
+            }
+            x => {
+                context.add_var(var_id, x.clone());
+
+                let result = eval_expression(&context, &cond)?;
+
+                if result.is_true() {
+                    Ok(x)
+                } else {
+                    Ok(Value::Nothing { span: call.head })
+                }
+            }
+        }
+    }
+}