write-script-rust

## CLI Commands Place scripts in a folder. After writing, tell the user which command fits what they want to do: - `wmill script preview <script_path>` — **default when iterating on a local script.** Runs the local file without deploying. - `wmill script run <path>` — runs the script **already deployed** in the workspace. Use only when the user explicitly wants to test the deployed version, not local edits. - `wmill generate-metadata` — regenerate the local `.script.yaml` (input schema) and `.lock` (resolved dependencies) for scripts you changed, and refresh their content hashes in `wmill-lock.yaml`. Local files only — **not** a deploy. See "Keep metadata in sync" below. - Deploy local changes to the workspace — via `git push` or `wmill sync push` depending on how the repo is wired (see the **Deploying** section in `AGENTS.wmill.md`). Only suggest/run a deploy when the user explicitly asks to deploy/publish/push — not when they say "run", "try", or "test". ### Preview vs run — choose by intent, not habit If the user says "run the script", "try it", "test it", "does it work" while there are **local edits to the script file**, use `script preview`. Do NOT push the script to then `script run` it — pushing is a deploy, and deploying just to test overwrites the workspace version with untested changes. Only use `script run` when: - The user explicitly says "run the deployed version" / "run what's on the server". - There is no local script being edited (you're just invoking an existing script). Only use `sync push` when: - The user explicitly asks to deploy, publish, push, or ship. - The preview has already validated the change and the user wants it in the workspace. ### Keep metadata in sync after editing `wmill-lock.yaml` tracks a content hash for each item. Editing a script's content — most importantly **adding or removing an import** or **changing `main`'s arguments** — invalidates that hash and leaves the `.lock`, the `.script.yaml` input schema, and the hash row out of date. Run `wmill generate-metadata` (scoped to what you touched) after such edits so the resolved lock, the auto-generated args UI (driven by `.script.yaml`), and `wmill-lock.yaml` all match the code. Leaving them stale produces spurious diffs in git-sync and CI. This only writes local files (it is **not** a deploy), but it re-resolves dependencies, so it can bump unpinned versions (the same as deploying from the UI; expected, not a bug). So by default offer it and run it once the user agrees, rather than running it silently after every edit — unless the project's `AGENTS.md` opts into running metadata automatically (see the "Keeping metadata in sync" preference there). Either way YOU run the command, not the user. After running it, diff the regenerated `.lock` / `.script.lock` files and tell the user which dependency versions changed (e.g. `requests 2.31.0 → 2.32.0`), so they can catch an unwanted bump before deploying — even under `Metadata: auto`, since it's information, not a confirmation gate. Pin versions in code to keep them fixed. With no path argument, `generate-metadata` regenerates only the items whose content hash drifted — not everything. Imports propagate: editing a script that others import marks every importer stale too, so a one-line change to a shared module can regenerate many locks (by design — their locks must reflect the imported code). If it touches more than you expect, run `wmill generate-metadata --dry-run` — it lists each stale item with a reason (`content changed` or `depends on <path>`) without changing anything — then narrow with a path argument (`wmill generate-metadata f/foo`) or `--strict-folder-boundaries`. If the on-disk `.lock` and `.script.yaml` are already correct and only `wmill-lock.yaml` needs its hashes refreshed (hash drift, or bootstrapping missing entries), use `wmill generate-metadata rehash` — it re-records hashes from disk with no backend round-trip and no dependency changes. ### After writing — offer to test, don't wait passively If the user hasn't already told you to run/test/preview the script, offer it as a one-sentence next step (e.g. "Want me to run `wmill script preview` with sample args?"). Do not present a multi-option menu. If the user already asked to test/run/try the script in their original request, skip the offer and just execute `wmill script preview <path> -d '<args>'` directly — pick plausible args from the script's declared parameters. The shape varies by language: `main(...)` for code languages, the SQL dialect's own placeholder syntax (`$1` for PostgreSQL, `?` for MySQL/Snowflake, `@P1` for MSSQL, `@name` for BigQuery, etc.), positional `$1`, `$2`, … for Bash, `param(...)` for PowerShell. `wmill script preview` does not deploy, but it still executes script code and may cause side effects; run it yourself when the user asked to test/preview (or after confirming that execution is intended). `wmill generate-metadata` does not deploy either — it only writes local files (locks, schemas, hashes) — but offer it before running (or run automatically if the project's `AGENTS.md` opts in), per "Keep metadata in sync" above. Deploying to the workspace (`git push` or `wmill sync push` depending on how the repo is wired — see the **Deploying** section) is the only step that mutates remote state — do it only when the user explicitly asks to deploy/publish/push. For a **visual** open-the-script-in-the-dev-page preview (rather than `script preview`'s run-and-print-result), use the `preview` skill. Use `wmill resource-type list --schema` to discover available resource types. # Windmill Script Writing Guide ## General Principles - A script's inputs are its parameters. Credentials and configuration come in as resource-typed parameters, never hard-coded or read from the environment; the language section below shows how that language declares parameters - Libraries are installed automatically - do not show installation instructions - In a language with an entrypoint function (TypeScript, Python, Go, Rust, PHP, R, …), name it `main` (`Main` in C#) and do not call it; in TypeScript it must be async. SQL, GraphQL, Bash, PowerShell and Ansible scripts have no `main`: their language section shows how they take arguments - Where the language has a Windmill client (`wmill`), use it to interact with the platform - A script's input schema may carry a top-level `prompt_for_ai` string: its author's instructions to an AI choosing the inputs. Follow it when you pick arguments to run that script, and keep it when you rewrite the schema ## Return Values - A script can return any JSON-serializable value; a SQL script returns the rows its query produces - Return values become available to subsequent flow steps via `results.step_id` ## Preprocessor Scripts Preprocessor scripts process raw trigger data from various sources (webhook, custom HTTP route, SQS, WebSocket, Kafka, NATS, MQTT, AMQP, Postgres, GCP Pub/Sub, Azure, or email) before passing it to the flow. This separates the trigger logic from the flow logic and keeps the auto-generated UI clean. A preprocessor is written in TypeScript or Python: its function is named `preprocessor` instead of `main`, and it receives a single parameter called `event` (the language section gives its type). The returned object determines the parameter values passed to the flow. e.g., `{ b: 1, a: 2 }` calls the flow with `a = 2` and `b = 1`, assuming the flow has two inputs called `a` and `b`. # Rust ## Structure The script must contain a function called `main` with proper return type: ```rust use anyhow::anyhow; use serde::Serialize; #[derive(Serialize, Debug)] struct ReturnType { result: String, count: i32, } fn main(param1: String, param2: i32) -> anyhow::Result<ReturnType> { Ok(ReturnType { result: param1, count: param2, }) } ``` **Important:** - Arguments should be owned types - Return type must be serializable (`#[derive(Serialize)]`) - Return type is `anyhow::Result<T>` ## Dependencies Packages must be specified with a partial cargo.toml at the beginning of the script: ```rust //! ```cargo //! [dependencies] //! anyhow = "1.0.86" //! reqwest = { version = "0.11", features = ["json"] } //! tokio = { version = "1", features = ["full"] } //! ``` use anyhow::anyhow; // ... rest of the code ``` **Note:** Serde is already included, no need to add it again. ## Async Functions If you need to handle async functions (e.g., using tokio), keep the main function sync and create the runtime inside: ```rust //! ```cargo //! [dependencies] //! anyhow = "1.0.86" //! tokio = { version = "1", features = ["full"] } //! reqwest = { version = "0.11", features = ["json"] } //! ``` use anyhow::anyhow; use serde::Serialize; #[derive(Serialize, Debug)] struct Response { data: String, } fn main(url: String) -> anyhow::Result<Response> { let rt = tokio::runtime::Runtime::new()?; rt.block_on(async { let resp = reqwest::get(&url).await?.text().await?; Ok(Response { data: resp }) }) } ```