refactor: update installable resolution to use fallback option

Author: NotAShelf

src/darwin.rs

@@ -72,8 +72,8 @@ impl DarwinRebuildArgs {
         debug!(?out_path);
 
         // Resolve the installable from env var or from the provided argument
-        let installable =
-            platform::resolve_env_installable("NH_DARWIN_FLAKE", self.common.installable.clone());
+        let installable = platform::resolve_env_installable("NH_DARWIN_FLAKE")
+            .unwrap_or_else(|| self.common.installable.clone());
 
         // Build the darwin configuration with proper attribute path handling
         let target_profile = platform::handle_rebuild_workflow(
@@ -151,7 +151,8 @@ impl DarwinReplArgs {
 
         // Open an interactive REPL session for exploring darwin configurations
         platform::run_repl(
-            platform::resolve_env_installable("NH_DARWIN_FLAKE", self.installable),
+            platform::resolve_env_installable("NH_DARWIN_FLAKE")
+                .unwrap_or_else(|| self.installable),
             "darwinConfigurations",
             &[], // REPL doesn't need additional path elements
             Some(hostname),

src/home.rs

@@ -67,8 +67,8 @@ impl HomeRebuildArgs {
         debug!(?out_path);
 
         // Use NH_HOME_FLAKE if available, otherwise use the provided installable
-        let installable =
-            platform::resolve_env_installable("NH_HOME_FLAKE", self.common.installable.clone());
+        let installable = platform::resolve_env_installable("NH_HOME_FLAKE")
+            .unwrap_or_else(|| self.common.installable.clone());
 
         // Set up the specialisation path
         let spec_location =
@@ -168,7 +168,8 @@ impl HomeReplArgs {
     /// debugging or exploring available options.
     fn run(self) -> Result<()> {
         // Use NH_HOME_FLAKE if available, otherwise use the provided installable
-        let installable = platform::resolve_env_installable("NH_HOME_FLAKE", self.installable);
+        let installable =
+            platform::resolve_env_installable("NH_HOME_FLAKE").unwrap_or_else(|| self.installable);
 
         // Launch an interactive REPL session for exploring the configuration
         platform::run_repl(

src/nixos.rs

@@ -36,19 +36,19 @@ impl interface::OsArgs {
     /// - `BuildVm`: Builds a `NixOS` VM image
     pub fn run(self) -> Result<()> {
         use OsRebuildVariant::{Boot, Build, Switch, Test};
-        // Always resolve installable from env var at the top
-        let resolved_installable = platform::resolve_env_installable(
-            "NH_OS_FLAKE",
-            match &self.subcommand {
-                OsSubcommand::Boot(args) => args.common.installable.clone(),
-                OsSubcommand::Test(args) => args.common.installable.clone(),
-                OsSubcommand::Switch(args) => args.common.installable.clone(),
-                OsSubcommand::Build(args) => args.common.installable.clone(),
-                OsSubcommand::BuildVm(args) => args.common.common.installable.clone(),
-                OsSubcommand::Repl(args) => args.installable.clone(),
-                _ => Installable::default(), // fallback for Info/Rollback, not used
-            },
-        );
+        // Always resolve installable from env var at the top, or use the provided one
+        let fallback_installable = match &self.subcommand {
+            OsSubcommand::Boot(args) => args.common.installable.clone(),
+            OsSubcommand::Test(args) => args.common.installable.clone(),
+            OsSubcommand::Switch(args) => args.common.installable.clone(),
+            OsSubcommand::Build(args) => args.common.installable.clone(),
+            OsSubcommand::BuildVm(args) => args.common.common.installable.clone(),
+            OsSubcommand::Repl(args) => args.installable.clone(),
+            _ => Installable::default(), // fallback for Info/Rollback, not used
+        };
+
+        let resolved_installable =
+            platform::resolve_env_installable("NH_OS_FLAKE").unwrap_or(fallback_installable);
         match self.subcommand {
             OsSubcommand::Boot(args) => {
                 args.rebuild_with_installable(Boot, None, resolved_installable)

src/util/platform.rs

@@ -10,9 +10,12 @@ use tracing::{debug, info, warn};
 use crate::commands;
 use crate::installable::Installable;
 
-/// Resolves an Installable from an environment variable, or falls back to the provided one.
-pub fn resolve_env_installable(var: &str, fallback: Installable) -> Installable {
-    if let Ok(val) = env::var(var) {
+/// Resolves an Installable from an environment variable.
+///
+/// Returns `Some(Installable)` if the environment variable is set and can be parsed,
+/// or `None` if the environment variable is not set.
+pub fn resolve_env_installable(var: &str) -> Option<Installable> {
+    env::var(var).ok().map(|val| {
         let mut elems = val.splitn(2, '#');
         let reference = elems.next().unwrap().to_owned();
         let attribute = elems
@@ -23,9 +26,7 @@ pub fn resolve_env_installable(var: &str, fallback: Installable) -> Installable
             reference,
             attribute,
         }
-    } else {
-        fallback
-    }
+    })
 }
 
 /// Extends an Installable with the appropriate attribute path for a platform.