Add repl subcommand

Fixes #115.

manual/colorized-help.nix

@@ -14,6 +14,7 @@ let
     "eval"
     "exec"
     "nix-info"
+    "repl"
   ];
   renderHelp = subcommand: let
     fullCommand = if subcommand == null then "colmena" else "colmena ${subcommand}";

manual/src/features/eval.md

@@ -30,3 +30,18 @@ You may directly instantiate an expression that evaluates to a derivation:
 $ colmena eval --instantiate -E '{ nodes, ... }: nodes.alpha.config.boot.kernelPackages.kernel'
 /nix/store/7ggmhnwvywrqcd1z2sdpan8afz55sw7z-linux-5.14.14.drv
 ```
+
+## Interactive REPL
+
+To explore the configurations interactively, start a REPL session with `colmena repl`:
+
+```console
+$ colmena repl
+[INFO ] Using flake: git+file:///home/user/cluster
+Welcome to Nix 2.10.3. Type :? for help.
+
+Loading installable ''...
+Added 3 variables.
+nix-repl> nodes.alpha.config.deployment.targetHost
+"fd12:3456::1"
+```

manual/src/release-notes.md

@@ -11,6 +11,7 @@
 - The [`meta.allowApplyAll`](./reference/meta.md#allowapplyall) option has been added. If set to false, deployments without a node filter (`--on`) are disallowed ([#95](https://github.com/zhaofengli/colmena/issues/95)).
 - The `--no-substitutes` option under the `apply` subcommand has been renamed to `--no-substitute` ([#59](https://github.com/zhaofengli/colmena/issues/59)).
 - The [`meta.nodeSpecialArgs`](./reference/meta.md#nodespecialargs) option has been added. It allows specifying node-specific `specialArgs` passed to NixOS modules ([#100](https://github.com/zhaofengli/colmena/pull/100)).
+- The [`repl`](./reference/cli.html#colmena-repl) subcommand has been added. It allows you to start an [interactive REPL](./features/eval.md#interactive-repl) with access to the complete node configurations.
 
 ## [Release 0.3.0](https://github.com/zhaofengli/colmena/releases/tag/v0.3.0) (2022/04/27)
 

src/cli.rs

@@ -171,6 +171,7 @@ It's also possible to specify the preference using environment variables. See <h
     register_command!(eval, app);
     register_command!(upload_keys, app);
     register_command!(exec, app);
+    register_command!(repl, app);
     register_command!(nix_info, app);
 
     // This does _not_ take the --color flag into account (haven't
@@ -196,6 +197,7 @@ pub async fn run() {
     handle_command!(eval, matches);
     handle_command!("upload-keys", upload_keys, matches);
     handle_command!(exec, matches);
+    handle_command!(repl, matches);
     handle_command!("nix-info", nix_info, matches);
 
     #[cfg(debug_assertions)]

src/command/mod.rs

@@ -3,6 +3,7 @@ pub mod build;
 pub mod eval;
 pub mod exec;
 pub mod nix_info;
+pub mod repl;
 pub mod upload_keys;
 
 #[cfg(target_os = "linux")]

src/command/repl.rs

@@ -0,0 +1,38 @@
+use clap::{ArgMatches, Command as ClapCommand};
+use tokio::process::Command;
+
+use crate::error::ColmenaError;
+use crate::util;
+
+pub fn subcommand() -> ClapCommand<'static> {
+    ClapCommand::new("repl")
+        .about("Start an interactive REPL with the complete configuration")
+        .long_about(
+            r#"Start an interactive REPL with the complete configuration
+
+In the REPL, you can inspect the configuration interactively with tab
+completion. The node configurations are accessible under the `nodes`
+attribute set."#,
+        )
+}
+
+pub async fn run(_global_args: &ArgMatches, local_args: &ArgMatches) -> Result<(), ColmenaError> {
+    let hive = util::hive_from_args(local_args).await?;
+
+    let expr = hive.get_repl_expression();
+
+    let status = Command::new("nix")
+        .arg("repl")
+        // `nix repl` is expected to be marked as experimental:
+        // <https://github.com/NixOS/nix/issues/5604>
+        .args(&["--experimental-features", "nix-command flakes"])
+        .args(&["--expr", &expr])
+        .status()
+        .await?;
+
+    if !status.success() {
+        return Err(status.into());
+    }
+
+    Ok(())
+}

src/nix/hive/mod.rs

@@ -369,6 +369,11 @@ impl Hive {
         }
     }
 
+    /// Returns the expression for a REPL session.
+    pub fn get_repl_expression(&self) -> String {
+        format!("{} hive.introspect (x: x)", self.get_base_expression())
+    }
+
     /// Returns the base expression from which the evaluated Hive can be used.
     fn get_base_expression(&self) -> String {
         self.assets.get_base_expression()