Draft: let? blog post

Author: fhammerschmidt

_blogposts/2025-09-01-let-unwrap.mdx

@@ -0,0 +1,75 @@
+---
+author: rescript-team
+date: "2025-09-01"
+badge: roadmap
+title: let?
+description: |
+  A new let-unwrap syntax just landed in ReScript.
+---
+
+After long discussions we finally decided on an unwrap syntax for both the `option` and `result` types that we are happy with and that still matches the explicitness of ReScript we all like.
+
+# What is it exactly?
+
+`let?` or `let-unwrap` is a tiny syntax that unwraps `result`/`option` values and *early-returns* on `Error`/`None`. It’s explicitly **experimental** and **disabled by default** behind a new “experimental features” gate.
+
+### Example
+
+```rescript
+let getUser = async (id) => {
+  let? Ok(user)  = await fetchUser(id)
+  let? Ok(decodedUser) = decodeUser(user)
+  Console.log(`Got user ${decodedUser.name}!`)
+  let? Ok() = await ensureUserActive(decodedUser)
+  Ok(decodedUser)
+}
+```
+
+This desugars to a **sequence** of `switch`/early-returns that you’d otherwise write by hand, so there’s **no extra runtime cost** and it plays nicely with `async/await`. Same idea works for `option` with `Some(...)` (and the PR also extends support so the left pattern can be `Error(...)`/`None`, not just `Ok(...)`/`Some(...)`). ([[ReScript Forum](https://forum.rescript-lang.org/t/proposing-new-syntax-for-zero-cost-unwrapping-options-results/6227)][2], [[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+
+### Where it works (and doesn’t)
+
+* Targets **built-ins** only: `result` and `option`. (Custom variants still need `switch`.) ([[ReScript Forum](https://forum.rescript-lang.org/t/proposing-new-syntax-for-zero-cost-unwrapping-options-results/6227)][2])
+* **Block/local bindings** only; tests show top-level usage is rejected. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+* The printed JS is the straightforward if/return form (i.e., “zero cost”). ([[ReScript Forum](https://forum.rescript-lang.org/t/proposing-new-syntax-for-zero-cost-unwrapping-options-results/6227)][2])
+
+### How to enable it (experimental)
+
+* The PR adds an **experimental-features infrastructure** to the toolchain and **CLI support**: `-enable-experimental …`. Rewatch reads config from `rescript.json` and forwards the feature(s) to the compiler. (See `rewatch` config + compile changes and the new compiler flag.) ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+* There are tests and docs stubs around the experimental toggle plus “feature not enabled” error cases. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+
+> In short: add the feature in `rescript.json` via the new “experimental features” setting (per the updated `CompilerConfigurationSpec.md`), or pass `-enable-experimental` in your build; otherwise `let?` is unavailable. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+
+# Dev notes (what changed under the hood)
+
+* **Lexer/Parser/Printer:** new token for `let?`, grammar rules, and pretty-printer support. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+* **Frontend transform:** AST handling lowers `let?` to the equivalent `switch`/early return; tailored super-errors for misuse. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+* **Feature gate:** new module(s) to register/enable experimental features, with CLI wiring and rewatch config reading. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+* **Tests:** syntax parsing, printer snapshots, super-errors (e.g., “feature not enabled”, “top-level not allowed”, “not supported variant”, and return-type mismatch). ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+
+# Safety analysis: can this break anything when **off**?
+
+**Bottom line:** extremely low risk when the flag is **off**.
+
+Why:
+
+1. **Gated by default.** The feature is unreachable unless explicitly enabled; using `let?` without the gate yields a dedicated “feature not enabled” error (there are fixtures for this). That means existing codebases remain unaffected. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+2. **New syntax, not repurposed.** `let?` didn’t previously parse; recognizing it only at a `let` binding start avoids collisions with existing `?` uses (e.g., ternary) elsewhere in expressions. Parser tests were added to lock this down. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+3. **No runtime path changes.** It’s a compile-time sugar that lowers to the same `switch`/return structure you’d hand-write. If you don’t use it, nothing in your generated JS changes. ([[ReScript Forum](https://forum.rescript-lang.org/t/proposing-new-syntax-for-zero-cost-unwrapping-options-results/6227)][2])
+4. **Confined scope.** Even when enabled, top-level `let?` is rejected; only local/block bindings are supported. This reduces any accidental global impact. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+5. **Tooling guarded.** The PR tracks TODOs for error coverage and editor support; with the gate off, current editor plugins behave as before. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582)][1])
+
+**Potential edge considerations (still low risk off):**
+
+* **Tokenization side-effects:** The scanner learns the `let?` token, but the feature gate + grammar location prevents it from altering how existing, valid programs lex/parse when you don’t write `let?`. Tests cover “not enabled” and misuse shapes. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+* **Error text churn:** Some super-error messages were added (including a hint when code looks *eligible* for `let?`). That only triggers in error paths; it won’t change successful builds. ([[GitHub](https://github.com/rescript-lang/rescript/pull/7582/files)][3])
+
+**Conclusion:** With the experimental flag **off**, there are no functional or runtime changes, and parser behavior for all previously valid code paths is preserved. It’s safe to ship behind the flag.
+
+---
+
+If you want, I can also jot down a tiny rollout checklist (enable flag in a sample app, verify editor plugin snapshot, run the super-errors suite) next.
+
+[1]: https://github.com/rescript-lang/rescript/pull/7582 "PoC of let? by zth · Pull Request #7582 · rescript-lang/rescript · GitHub"
+[2]: https://forum.rescript-lang.org/t/proposing-new-syntax-for-zero-cost-unwrapping-options-results/6227 "Proposing new syntax for zero-cost unwrapping options/results - Development - ReScript Forum"
+[3]: https://github.com/rescript-lang/rescript/pull/7582/files "PoC of let? by zth · Pull Request #7582 · rescript-lang/rescript · GitHub"