Docs / Language Manual / Migrate to ReScript Syntax

Migrate from BuckleScript/Reason

ReScript is a rebranding and cleanup of BuckleScript (since v8.2.0) & Reason (v3.6) that enables us to ship a tighter compile-to-JS stack with more coherent documentation & tools. If you're an existing user of BuckleScript & Reason, here's the gist:

  • ReScript is mostly just BuckleScript rebranded, with a new syntax that's like the Reason syntax, but catered more toward the JavaScript crowd.

  • All your existing code will keep working even if you don't upgrade.

Upgrade Your Codebase

There are lots of exciting improvements in the new syntax (features, speed, error messages, etc.). The upgrade is trivial, backward-compatible and can be done on a per-file basis:

npm install rescript@9 # Also works with .rei / .ml / .mli / etc npx rescript convert src/

Enjoy the improved experience!

Upgrade Individual Folders

This is useful for per-directory conversions:

# *.rei, *.ml, *.mli,.... for f in your-folder/**/*.re; do; node_modules/.bin/rescript convert $f && rm $f; done;

Upgrade an Entire Codebase

In case you are confident in converting your codebase in one step, use the following:

npx rescript convert -all

Difference With Old Reason

  • Complete removal of semicolon (you can still write them).

  • No need for parentheses around if, switch and try.

  • Type arguments: from option(int) to option<int>.

  • Old interpolated string: from {j|hello ${name}|j} to `hello ${name}`. Now with proper unicode support!

  • New interpolated string: `hello world`. Also supports multiline and unicode. "hello world" string is now singleline.

  • Polymorphic variants: from `red to #red.

  • Arrays: from [|1,2,3|] to [1,2,3]. In JS, arrays are the right default.

  • Lists: from [1,2,3] to list[1,2,3] (8.1.1 update: now it is list{1, 2, 3}). This ties with upcoming plans to access containers in a uniform way: set[...] and map[...]. Maybe temporary.

  • Exception: from try (compute()) { | Not_found => Js.log("oops")} to try compute() catch { | Not_found => Js.log("oops")}.

  • First class module: from (module S: Student) to module(S: Student).

  • No custom infix operator for now (including mod).

  • Object access: from settings##visible #= true to settings["visible"] = true. Rejoice!

  • Object: from Js.t({"age": int}) to just {"age": int}. The Js.t part is now unneeded.

  • Attribute: from [@myAttribute "hello"] to @myAttribute("hello"). From [%re bla] to %re(bla).

  • Removed dereference syntax result^. Just use result.contents.

  • fun pattern matching syntax removed.

  • Type declaration is non-recursive by default, consistent with let bindings. To have recursive types, use type rec myList<'a> = Nil | Cons('a, myList<'a>).

  • Use any words, including reserved keywords, as your identifier name: let \"try" = true.