Docs / Language Manual / ConvertingFromOcaml

Converting from OCaml

Converting a project from OCaml to Reason is straightforward and doesn't require semantic changes. However, there are a few build setup and miscellaneous changes required.


If your build system executes explicit build commands, then the easiest way to use Reason with ocamlopt/ocamlc is by adding the following flags to each compilation step:

# intf-suffix tells the compiler where to look for corresponding interface files ocamlopt -pp refmt -intf-suffix rei -impl ocamlopt -pp refmt -intf myFile.rei


There is no official Reason Syntax support for ocamlbuild though you can write custom rules for ocamlbuild that will use .re/.rei files correctly. This diff removes a legacy utility which can be used as a reference.


If your project uses Dune then Reason syntax is automatically supported. Add a dependency on the Reason package then Dune will pay attention to .re/.rei files in your project.

# If using esy esy add @esy-ocaml/reason # If using opam opam install reason

Converting From OCaml

OCaml code may be (mostly) automatically converted to Reason by using refmt. See refmt --help for more options:

refmt --parse ml --print re > refmt --parse ml --print re yourFile.mli > yourFile.rei

Cleaning Up The Output: The converted Reason code may attach [@implicit_arity] to variant constructors, like so: [@implicit_arity]C(1, 2). This is due to the fact that OCaml has the ambiguous syntax where a multi-arguments constructor is expecting argument in a tuple form. So it isn't clear if OCaml's C (1, 2) should be translated to Reason Syntax's C (1, 2) or C 1 2. By default, refmt will convert it to [@implicit_arity]C(1, 2), which tells the compiler this can be either C(1, 2) or C((1, 2)). The "implicit arity" tag ensures the output recreates OCaml's semantics so that it compiles correctly once converted to Reason.

You can easily clean up the refmt output by requesting that [@implicit_arity] tags be stripped. Pass the --assume-explicit-arity to refmt to force the formatter to generate C(1, 2) instead of [@implicit_arity]C(1,2). This approach will generate cleaner Reason Syntax but you may need to selectively convert some C(1,2) variants to C((1,2)) to make the type checker happy.

There are already some internal exception rules to cover the common constructors that require a single tuple as argument so refmt will be convert them correctly without requiring your manual cleanup (e.g., Some (1, 2) in OCaml will be converted to Some((1, 2)) instead of Some(1, 2)).

To provide your own exception list, create a line-separated file that contains all constructors (without module prefix) in your project that expects a single tuple as argument, and use --heuristics-file <filename> to tell refmt that all constructors listed in the file will be treated as constructor with a single tuple as argument:

> cat heuristics.txt TupleConstructor And Or
> cat
type tm = TupleConstructor of (int * int) | MultiArgumentsConstructor of int * int let x = TupleConstructor(1, 2) let y = MultiArgumentsConstructor(1, 2) module Test = struct type a = | And of (int * int) | Or of (int * int) end;; let a = Test.And (1, 2) let b = Test.Or (1, 2) let c = Some (1, 2)

Then only the constructor identifiers that were listed will be assumed to accept tuples instead of multiple arguments.

> refmt --heuristics-file \ ./heuristics.txt --assume-explicit-arity \ --parse ml --print re
type tm = | TupleConstructor((int, int)) | MultiArgumentsConstructor(int, int); let x = TupleConstructor((1, 2)); let y = MultiArgumentsConstructor(1, 2); module Test = { type a = | And((int, int)) | Or((int, int)); }; let a = Test.And((1, 2)); let b = Test.Or((1, 2)); let c = Some((1, 2));