Docs / Language Manual / Migrate to ReScript Syntax

You are currently looking at the < v8.2.0 docs (Reason v3.6 syntax edition). You can find the latest manual page here.

(These docs cover all versions between v3 to v8 and are equivalent to the old BuckleScript docs before the rebrand)

Migrate to New Syntax

Starting from v8.2.0, BuckleScript and the JS part of Reason are now fused into the new ReScript. We've also introduced a new syntax similar to the old Reason syntax, but tremendously improved.

The old ML and Reason syntax are still supported (see our support commitment here). The gist is: this is mostly just a rebranding and shouldn't affect your existing code much.

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:

  • Upgrade your project to bs-platform 8.2.0 or later.

  • node_modules/.bin/bsc -format > MyFile.res

That's it! could be any .re, .rei, .ml and .mli file.

Enjoy the improved experience!

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 j`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 implicit.

  • Attribute: from [@bs.deriving accessors] to @bs.deriving(accessors). 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.