introspect: Support actually instantiating the expression

src/command/introspect.rs

@@ -25,6 +25,10 @@ For example, to retrieve the configuration of one node, you may write something
             .value_name("EXPRESSION")
             .help("The Nix expression")
             .takes_value(true))
+        .arg(Arg::with_name("instantiate")
+            .long("instantiate")
+            .help("Actually instantiate the expression")
+            .takes_value(false))
 }
 
 pub async fn run(_global_args: &ArgMatches<'_>, local_args: &ArgMatches<'_>) {
@@ -42,6 +46,12 @@ pub async fn run(_global_args: &ArgMatches<'_>, local_args: &ArgMatches<'_>) {
         format!("import {}", path.canonicalize().expect("Could not generate absolute path to expression file.").to_str().unwrap())
     };
 
-    let result = hive.introspect(expression).await.unwrap();
+    let instantiate = local_args.is_present("instantiate");
+    let result = hive.introspect(expression, instantiate).await.unwrap();
+
+    if instantiate {
+        print!("{}", result);
+    } else {
         println!("{}", result);
+    }
 }

src/nix/hive.rs

@@ -185,11 +185,17 @@ impl Hive {
     }
 
     /// Evaluates an expression using values from the configuration
-    pub async fn introspect(&self, expression: String) -> NixResult<String> {
+    pub async fn introspect(&self, expression: String, instantiate: bool) -> NixResult<String> {
+        if instantiate {
+            let expression = format!("hive.introspect ({})", expression);
+            self.nix_instantiate(&expression).instantiate_with_builders().await?
+                .capture_output().await
+        } else {
             let expression = format!("toJSON (hive.introspect ({}))", expression);
             self.nix_instantiate(&expression).eval_with_builders().await?
                 .capture_json().await
         }
+    }
 
     /// Retrieve machinesFile setting for the hive.
     async fn machines_file(&self) -> NixResult<Option<String>> {