Bare sausage creates ./world if needed and runs it (default app)

- cli: the subcommand is optional; without one, sausage runs ./world and
  runs init first when ./world is not a world yet (test: cli/tests/cli.rs).
  So `nix run git+https://gitea.surma.technology/surma/sausage` works.
- flake: .#loop without a folder runs bare sausage.
- README: mention the bare command and the bare loop.

README.md

@@ -14,7 +14,15 @@ You need [Nix](https://nixos.org/download) with flakes, and an API token for
 an LLM (OpenAI Responses or Anthropic Messages API). No clone needed:
 
 ```sh
-nix run git+https://gitea.surma.technology/surma/sausage -- init   # creates ./world and builds the agent (a few minutes)
+nix run git+https://gitea.surma.technology/surma/sausage
+```
+
+This creates a world in `./world` the first time (it builds the agent, which
+takes a few minutes), then starts it with a console. Later runs start the
+existing world. The two steps on their own:
+
+```sh
+nix run git+https://gitea.surma.technology/surma/sausage -- init   # creates ./world and builds the agent
 nix run git+https://gitea.surma.technology/surma/sausage -- run    # starts the world and a console
 ```
 
@@ -27,7 +35,8 @@ folder as an argument, for example `... -- run my-world`.
 To keep a world on the newest version, run it in a loop:
 
 ```sh
-nix run git+https://gitea.surma.technology/surma/sausage#loop -- my-world
+nix run git+https://gitea.surma.technology/surma/sausage#loop               # ./world, created if needed
+nix run git+https://gitea.surma.technology/surma/sausage#loop -- my-world   # an existing world
 ```
 
 The loop runs `nix run --refresh git+https://gitea.surma.technology/surma/sausage -- run my-world`.
@@ -36,7 +45,8 @@ fetches the newest commit and starts it again. `/quit` ends the loop. Set
 `SAUSAGE_FLAKE` to run another flake URL instead, for example your fork.
 
 From a clone (`git clone https://gitea.surma.technology/surma/sausage`), the
-same commands are `nix run -- init`, `nix run -- run` and `nix run .#loop`.
+same commands are `nix run`, `nix run -- init`, `nix run -- run` and
+`nix run .#loop`.
 
 In the console:
 

cli/src/main.rs

@@ -12,11 +12,13 @@ use sausage::{Host, cargo_actor, console};
 /// The source of the agent actor. `init` builds it like any other actor.
 static AGENT_SOURCE: Dir<'_> = include_dir!("$CARGO_MANIFEST_DIR/../agent");
 
+// Without a command, `sausage` runs the world in `./world` and creates it
+// first (like `init`) if it does not exist yet.
 #[derive(Parser)]
 #[command(version, about)]
 struct Cli {
     #[command(subcommand)]
-    command: Cmd,
+    command: Option<Cmd>,
 }
 
 #[derive(Subcommand)]
@@ -112,7 +114,15 @@ async fn run(dir: PathBuf) -> Result<()> {
 #[tokio::main]
 async fn main() -> Result<()> {
     match Cli::parse().command {
-        Cmd::Init { dir } => init(dir).await,
+        Some(Cmd::Init { dir }) => init(dir).await,
-        Cmd::Run { dir } => run(dir).await,
+        Some(Cmd::Run { dir }) => run(dir).await,
+        None => {
+            let dir = PathBuf::from("world");
+            if !dir.join("actor").is_dir() {
+                println!("No world in ./world yet, creating it (same as `sausage init`).");
+                init(dir.clone()).await?;
+            }
+            run(dir).await
+        }
     }
 }

cli/tests/cli.rs

@@ -0,0 +1,41 @@
+//! Tests of the `sausage` command line.
+
+use std::path::PathBuf;
+use std::process::{Command, Stdio};
+use std::time::{SystemTime, UNIX_EPOCH};
+
+fn empty_dir(label: &str) -> PathBuf {
+    let nonce = SystemTime::now().duration_since(UNIX_EPOCH).unwrap().as_nanos();
+    let dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .parent()
+        .unwrap()
+        .join("target/test-cli")
+        .join(format!("{label}-{}-{nonce}", std::process::id()));
+    std::fs::create_dir_all(&dir).unwrap();
+    dir
+}
+
+/// Bare `sausage` (as in `nix run <flake>`) without a world starts `init`
+/// for ./world instead of printing a usage error.
+#[test]
+fn bare_command_without_a_world_starts_init() {
+    let dir = empty_dir("bare");
+    let output = Command::new(env!("CARGO_BIN_EXE_sausage"))
+        .current_dir(&dir)
+        .env_remove("SAUSAGE_API")
+        .env_remove("SAUSAGE_BASE_URL")
+        .env_remove("SAUSAGE_MODEL")
+        .env_remove("SAUSAGE_API_TOKEN")
+        .stdin(Stdio::null())
+        .output()
+        .unwrap();
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    // With no answers on stdin, init stops at the first required value.
+    assert!(!output.status.success(), "stdout:\n{stdout}\nstderr:\n{stderr}");
+    assert!(stdout.contains("No world in ./world yet"), "stdout:\n{stdout}\nstderr:\n{stderr}");
+    assert!(stderr.contains("SAUSAGE_BASE_URL"), "stdout:\n{stdout}\nstderr:\n{stderr}");
+    assert!(!stderr.contains("Usage:"), "stderr:\n{stderr}");
+    // It asks before it creates anything.
+    assert!(!dir.join("world").exists());
+}

flake.nix

@@ -36,6 +36,7 @@
           meta.mainProgram = "sausage";
         };
         # `nix run .#loop -- <world>` runs the newest `sausage run <world>`
+        # (without a folder: bare `sausage`, i.e. ./world, created if needed)
         # from the public repo, again and again: after a crash or a broker
         # `exit` request it refreshes the flake and starts the host again,
         # so the world updates itself. `/quit` (exit code 0) ends the loop.
@@ -44,9 +45,10 @@
         repoUrl = "git+https://gitea.surma.technology/surma/sausage";
         loop = pkgs.writeShellScriptBin "sausage-loop" ''
           url="''${SAUSAGE_FLAKE:-${repoUrl}}"
-          if [ $# -eq 0 ]; then set -- world; fi
+          # No folder: bare `sausage` (creates ./world first if needed).
+          if [ $# -gt 0 ]; then set -- run "$@"; fi
           while true; do
-            nix run --refresh "$url#default" -- run "$@"
+            nix run --refresh "$url#default" -- "$@"
             code=$?
             if [ "$code" -eq 0 ]; then exit 0; fi
             echo "sausage-loop: sausage exited with $code, starting the newest version in 2 s …" >&2