Pipeline blocks (#1579)

* Making Commands match what UntaggedValue needs

* WIP

* WIP

* WIP

* Moved to expressions for conditions

* Add 'each' command to use command blocks

* More cleanup

* Add test for 'each'

* Instead use an expression block

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

@@ -6,7 +6,7 @@ use crate::prelude::*;
 use derive_new::new;
 use getset::Getters;
 use nu_errors::ShellError;
-use nu_parser::hir;
+use nu_protocol::hir;
 use nu_protocol::{CallInfo, EvaluatedArgs, ReturnValue, Scope, Signature, Value};
 use serde::{Deserialize, Serialize};
 use std::ops::Deref;
@@ -364,6 +364,14 @@ impl EvaluatedCommandArgs {
         self.call_info.args.nth(pos)
     }
 
+    /// Get the nth positional argument, error if not possible
+    pub fn expect_nth(&self, pos: usize) -> Result<&Value, ShellError> {
+        match self.call_info.args.nth(pos) {
+            None => Err(ShellError::unimplemented("Better error: expect_nth")),
+            Some(item) => Ok(item),
+        }
+    }
+
     pub fn get(&self, name: &str) -> Option<&Value> {
         self.call_info.args.get(name)
     }