cli: add subcommand support

crates/app/src/main.rs

@@ -1,6 +1,7 @@
-use std::fs;
+use std::{fs, path::PathBuf};
 
-use config::Config;
+use clap::{Parser, Subcommand};
+use config::{CliConfigs, Config};
 use welcome_msg::print_startup_msg;
 
 mod config;
@@ -9,19 +10,44 @@ mod config;
 mod error_reporting;
 mod welcome_msg;
 
+#[derive(Parser)]
+struct Args {
+    #[command(flatten)]
+    configs: CliConfigs,
+    #[command(subcommand)]
+    command: Commands,
+}
+
+#[derive(Subcommand)]
+enum Commands {
+    Run {
+        /// What file contains the pipeline to evaluate.
+        source: PathBuf,
+    },
+    Dev,
+}
+
 fn main() {
     // TODO: proper error handling across the whole function
     //       don't forget to also look inside `Config`
-    let cfg = Config::read();
+    let args = Args::parse();
+    let cfg = Config::read(&args.configs);
 
     if cfg.startup_msg {
         print_startup_msg();
     }
 
-    let source = fs::read_to_string(cfg.source).expect("can't find source file");
-    let ir = ir::from_ron(&source).expect("failed to parse source to graph ir");
+    match args.command {
+        Commands::Run { source } => {
+            let source = fs::read_to_string(source).expect("can't find source file");
+            let ir = ir::from_ron(&source).expect("failed to parse source to graph ir");
 
-    let mut machine = cfg.evaluator.pick();
-    machine.feed(ir);
-    machine.eval_full();
+            let mut machine = cfg.evaluator.pick();
+            machine.feed(ir);
+            machine.eval_full();
+        }
+        Commands::Dev => {
+            println!("Hello world!");
+        }
+    }
 }