Reason genType

Status master (v3.*):

genType lets you export Reason values and types to use in JavaScript, and import JavaScript values and types into Reason, idiomatically. Converter functions between the two representations are generated based on the type of the value. The converters can be generated in vanilla JavaScript, or in TypeScript / Flow for a type-safe idiomatic interface.
In particular, conversion of ReasonReact components both ways is supported, with automatic generation of the wrappers.

Here's an article describing how to use genType as part of a migration strategy where a tree of components is gradually converted to Reason bottom-up: Adopting Reason: strategies, dual sources of truth, and why genType is a big deal.

The implementation of [@genType] performs a type-directed transformation of Reason programs after bucklescript compilation. The transformed programs operate on data types idiomatic to JS. For example, a Reason function operating on a Reason variant type t =, A(int), B(string) (which is represented as custom blocks at runtime) is exported to a JS function operating on the corresponding JS object of type { tag: "A"; value: number }, { tag: "B"; value: string }.

The output of genType can be configured by using one of 3 back-ends: untyped to generate wrappers in vanilla JS, typescript to generate TypeScript, and flow to generate JS with Flow type annotations.

Project status.

See Changes.md for a complete list of features, fixes, and changes for each release.

Requirements

bs-platform 7.0.2 or higher: use genType 3.8.0 or higher.

bs-platform 7.0.0 or higher: use genType 3.2.0 or higher.

bs-platform 6.2.0 or higher: use genType 3.0.0 or higher.

bs-platform 5.2.0 or higher: use genType 2.40.0 or higher.

bs-platform 5.0.x and 5.1.x: use genType 2.17.0 or higher.

For earlier versions, see the older README.

Installation

Install the binaries via npm:

npm install --save-dev gentype

# Test running gentype
npx gentype --help

Add a gentypeconfig section to your bsconfig.json (See Configuration for details):

"gentypeconfig": {
    "language": "untyped",
    "shims": {},
    "debug": {
      "all": false,
      "basic": false
    }
}

For running gentype with BuckleScript via npm workflow, add following script in your package.json:

scripts: {
  "bs:build": "bsb -make-world",
  "bs:clean": "bsb -clean-world"
}

Note: With genType < 2.17.0 or bucklescript < 5.0.0, one has to set environment variable BS_CMT_POST_PROCESS_CMD. See the older README.

With this configuration, BuckleScript will call gentype for each newly built file. You might want to clean your build artifacts before usage: npx bsb -clean-world (otherwise there might be cached values and no .re.js files are generated).

Check out the Examples for detailed setups (TypeScript, Flow and Plain JavaScript).

Adding shims (TypeScript & Flow)

Configure your shim files in your "gentypeconfig" in bsconfig.json, and add relevant .shims.js files in a directory which is visible by bucklescript e.g. src/shims/. An example shim to export ReactEvent can be found here.

Testing the whole setup

Open any relevant *.re file and add [@genType] annotations to any bindings / values / functions to be used from JavaScript. If an annotated value uses a type, the type must be annotated too. See e.g. Hooks.re.

Save the file and rebuild the project with BuckleScript. You should now see a *.gen.tsx (for TypeScript, or *.gen.js for Flow) file with the same name (e.g. MyComponent.re -> MyComponent.gen.tsx).

Any values exported from MyComponent.re can then be imported from JS. For example:

import MyComponent from "./components/MyComponent.gen";

Examples

We prepared some examples to give you an idea on how to integrate genType in your own project. Check out the READMEs of the listed projects.

Please make sure to build genType before trying to build the examples.

• flow-react-example
• typescript-react-example
• untyped-react-example

Documentation

genType operates on two kinds of entities: types and values.
Each can be exported from Reason to JS, or imported into Reason from JS.
The main annotation is @genType, which by default means export.

Export and Import Types

The following exports a function type callback to JS:

[@genType]
type callback = ReactEvent.Mouse.t => unit;

To instead import a type called complexNumber from JS module MyMath.ts (or MyMath.js), use the @genType.import annotation:

[@genType.import "./MyMath"]
type complexNumber;

This imported type will be treated as opaque by Reason.

Export and Import Values

To export a function callback to JS:

[@genType]
let callback = _ => Js.log("Clicked");

To rename the function and export it as CB on the JS side, use

[@genType]
[@genType.as "CB"]
let callback = _ => Js.log("Clicked");

or the more compact

[@genType "CB"]
let callback = _ => Js.log("Clicked");

To import a function realValue from JS module MyMath.ts (or MyMath.js):

[@genType.import "./MyMath"] /* JS module to import from. */
/* Name and type of the JS value to import. */
external realValue: complexNumber => float = "";

Note: With genType < 2.17.0 or bucklescript < 5.0.0, one had to add a line with @bs.module and the current file name. See the older README.

Because of the external keyword, it's clear from context that this is an import, so you can also just use @genType and omit .import.

To import a default JS export, use a second argument to @genType.import e.g. [@genType.import ("./MyMath", "default")].

Similarly, to import a value with a different JS name, use e.g. [@genType.import ("./MyMath", "ValueStartingWithUpperCaseLetter")].

To import nested values, e.g. Some.Nested.value, use e.g. [@genType.import ("./MyMath", "Some.Nested.value")].

Export and Import React Components Using Deprecated Record API

To export a ReasonReact component to JS, and automatically generate a wrapper for it, simply annotate the make function:

[@genType]
let make = (~onClick: callback, _children) => {
  ...component,
  render: _ => <div onClick> "Click me"->ReasonReact.string </div>,
};

NOTE the value component must also be defined, above make in the same module (also in the case of components defined in nested modules).

To import and wrap a ReactJS component for use by ReasonReact, the type of the make function is the only information required:

[@genType.import "./MyBanner"] /* Module with the JS component to be wrapped. */
/* The make function will be automatically generated from the types below. */
external make:
  (~show: bool, ~message: option(message)=?, 'a) =>
  ReasonReact.component(
    ReasonReact.stateless,
    ReasonReact.noRetainedProps,
    ReasonReact.actionless,
  ) =
  "";

The type of make must have a named argument for each prop in the JS component. Optional props have option type. The make function will be generated by genType.

This assumes that the JS component was exported with a default export. In case of named export, use e.g. [@genType.import ("./MyBanner", "componentName")]. To import a nested component, use e.g. [@genType.import ("./MyBanner", "Some.Nested.component")].

Interface (.rei) and Implementation (.re) files

If both Foo.rei and Foo.re exist, the annotations are taken from Foo.rei.

The behaviour can be overridden by adding annotation [@genType.ignoreInterface]; at the top of Foo.rei. Use case: expose implementation details to JS but not to Reason.

Type Expansion and @genType.opaque

If an exported type persons references other types in its definition, those types are also exported by default, as long as they are defined in the same file:

type name = string;
type surname = string;
type person = {name:name, surname:surname};

[@genType]
type persons = array(person);

If however you wish to hide from JS the fact that name and surname are strings, you can do it with the @genType.opaque annotation:

[@genType.opaque]
type name = string;
[@genType.opaque]
type surname = string;

type person = {
  name,
  surname,
};

[@genType]
type persons = array(person);

Renaming, @genType.as, and object mangling convention.

By default, entities with a given name are exported/imported with the same name. However, you might wish to change the appearence of the name on the JS side.

NOTE From bucklescript 7.0.0, @genType.as on record fields will be discouraged,
as it incurs a runtime conversion cost. Instead @bs.as will be supported and incur zero cost.

For example, in case of a record field whose name is a keyword, such as type:

[@genType]
type shipment = {
  date: float,
  [@genType.as "type"]
  type_: string,
};

Object field names follow bucklescript's mangling convention:

Remove trailing "__" if present.
Otherwise remove leading "_" when followed by an uppercase letter, or keyword.

This means that the analogous example with objects is:

[@genType]
type shipment = {
  .
  "date": float,
  "_type": string,
};

or the equivalent "type__": string.

Functions and function components also follow the mangling convention for labeled arguments:

[@genType]
let exampleFunction = (~_type) => "type: " ++ _type;

[@genType]
[@react.component]
let exampleComponent = (~_type) => React.string("type: " ++ _type);

It is possible to use @genType.as for functions, though this is only maintained for backwards compatibility, and cannot be used on function components:

[@genType]
let functionWithGenTypeAs =
  (~date: float) => [@genType.as "type"] (~type_: string) => ...

NOTE For technical reasons, it is not possible to use @genType.as on the first argument of a function (restriction lifted on OCaml 4.0.6).

Dependent projects/libraries

Bucklescript dependencies are specified in bs-dependencies.
For example, if the dependencies are "bs-dependencies": ["somelibrary"] and somelibrary contains Common.re, this looks up the types of foo in the library:

  [@genType]
  let z = Common.foo;

Scoped packages of the form e.g. @demo/somelibrary are also supported.

NOTE The library must have been published with the .gen.ts files created by genType.

Configuration

Every genType powered project requires a configuration item "gentypeconfig" at top level in the project's bsconfig.json. (The use of a separate file gentypeconfig.json is being deprecated). The option has following structure:

...
  "gentypeconfig": {
    "language": "typescript", "flow", "untyped",
    "shims": {
      "ReasonReact": "ReactShim"
    }
  }

• language

  • "typescript" : Generate *.tsx files written in TypeScript.
  • "flow": Generate *.re.js files with Flow type annotations.
  • "untyped": Generate *.re.js files in vanilla JavaScript.

• shims

  • e.g. Array<string> with format: "ReasonModule=JavaScriptModule"
  • Required to export certain basic Reason data types to JS when one cannot modify the sources to add annotations (e.g. exporting Reason lists)

Types Supported

int

Reason values e.g. 1, 2, 3 are unchanged. So they are exported to JS values of type number.

float

Reason values e.g. 1.0, 2.0, 3.0 are unchanged. So they are exported to JS values of type number.

string

Reason values e.g. "a", "b", "c" are unchanged. So they are exported to JS values of type string.

optionals

Reason values of type e.g. option(int), such as None, Some(0), Some(1), Some(2), are exported to JS values null, undefined, 0, 1, 2.
The JS values are unboxed, and null/undefined are conflated.
So the option type is exported to JS type null or undefined or number.

nullables

Reason values of type e.g. Js.Nullable.t(int), such as Js.Nullable.null, Js.Nullable.undefined, Js.Nullable.return(0), Js.Nullable.return(1), Js.Nullable.return(2), are exported to JS values null, undefined, 0, 1, 2.
The JS values are identical: there is no conversion unless the argument type needs conversion.

records

Reason record values of type e.g. {x:int} such as {x:0}, {x:1}, {x:2}, are exported to JS object values {x:0}, {x:1}, {x:2}. This requires a change of runtime representation from arrays to objects.
So they are exported to JS values of type {x:number}.

Since records are immutable by default, their fields will be exported to readonly property types in Flow/TS. Mutable fields are specified in Reason by e.g. {mutable mutableField: string}.

The @genType.as annotation can be used to change the name of a field on the JS side of things. So e.g. {[@genType.as "y"] x:int} is exported as JS type {y:int}.

If one field of the Reason record has option type, this is exported to an optional JS field. So for example Reason type {x: option(int)} is exported as JS type {x?: number}.

objects

Reason object values of type e.g. {. "x":int} such as {"x": 0}, {"x": 1}, {"x": 2}, are exported as identical JS object values {x:0}, {x:1}, {x:2}. This requires no conversion. So they are exported to JS values of type {x:number}.
A conversion is required only when the type of some field requires conversions.

Since objects are immutable by default, their fields will be exported to readonly property types in Flow/TS. Mutable fields are specified in Reason by e.g. {. [@bs.set] "mutableField": string }.

It is possible to mix object and option types, so for example the Reason type {. "x":int, "y":option(string)} exports to JS type {x:number, ?y: string}, requires no conversion, and allows option pattern matching on the Reason side.

Object field names follow bucklescript's mangling convention (so e.g. _type in Reason represents type in JS):

Remove trailing "__" if present.
Otherwise remove leading "_" when followed by an uppercase letter, or keyword.

tuples

Reason tuple values of type e.g. (int, string) are exported as identical JS values of type [number, string]. This requires no conversion, unless one of types of the tuple items does.
While the type of Reason tuples is immutable, there's currently no mature enforcement in TS/Flow, so they're currenty exported to mutable tuples.

variants

Ordinary variants (with capitalized cases, e.g. , A, B(int) ) and polymorphic variants (with a backtick, e.g. , `A, `B(int) ) are represented in the same way, so there's no difference from the point of view of JavaScript. Polymorphic variants don't have to be capitalized.

Variants can have an unboxed, or a boxed representation. The unboxed representation is used when there is at most one case with a payload, and that payload has object type; otherwise, a boxed representation is used. Object types are arrays, objects, records and tuples.

Variants without payloads are essentially sequences of identifiers.
E.g. type [@genType] type days =, Monday, Tuesday.
The corresponding JS representation is "Monday", "Tuesday".
Similarly, polymorphic variant type [@genType] type days = [, `Monday, `Tuesday ] has the same JS representation.

When at most one variant case has a payload, and if the payload is of object type, e.g.
[, Unnamed, Named({. "name": string, "surname": string}) ]
then the representation is unboxed: JS values are e.g. "Unnamed" and
{name: "hello", surname: "world"}. Similarly for polymorphic variants.
Note that this unboxed representation does not use the label "Named" of the variant case with payload, because that value is distinguished from the other payload-less cases by its type: an object.

If there is more than one case with payload, or if the single payload has not type object, a boxed representation is used. The boxed representation has shape {tag: "someTag", value: someValue}.
For example, type , A, B(int), C(string) has values such as "A" and
{tag: "B", value: 42} and {tag: "C", value: "hello"}.
Polymorhphic variants are treated similarly. Notice that payloads for polymorphic variants are always unary: `Pair(int,int) has a single payload of type (int,int). Instead, ordinary variants distinguish between unary Pair((int,int)) and binary Pair(int,int) payloads. All those cases are represented in JS as {tag: "Pair", value: [3, 4]}, and the conversion functions take care of the different Reason representations.

The @genType.as annotation can be used to modify the name emitted for a variant case on the JS side. So e.g. , [@genType.as "Arenamed"] A exports Reason value A to JS value "Arenamed".
Boolean/integer/float constants can be expressed as , [@genType.as true] True and , [@genType.as 20] Twenty and , [@genType.as 0.5] Half . Similarly for polymorphic variants.
The @genType.as annotation can also be used on variants with payloads to determine what appears in { tag: ... }.

For more examples, see Variants.re and VariantsWithPayload.re.

NOTE When exporting/importing values that have polymorphic variant type, you have to use type annotations, and cannot rely on type inference. So instead of let monday = `Monday, use let monday : days = `Monday. The former does not work, as the type checker infers a type without annotations.

arrays

Arrays with elements of Reason type t are exported to JS arrays with elements of the corresponding JS type. If a conversion is required, a copy of the array is performed.

Immutable arrays are supported with the additional Reason library
ImmutableArray.re/.rei, which currently needs to be added to your project.
The type ImmutableArray.t(+'a) is covariant, and is mapped to readonly array types in TS/Flow. As opposed to TS/Flow, ImmutableArray.t does not allow casting in either direction with normal arrays. Instead, a copy must be performed using fromArray and toArray.

functions and function components

Reason functions are exported as JS functions of the corresponding type.
So for example a Reason function foo : int => int is exported as a JS function from numbers to numbers.

If named arguments are present in the Reason type, they are grouped and exported as JS objects. For example foo : (~x:int, ~y:int) => int is exported as a JS function from objects of type {x:number, y:number} to numbers.

In case of mixed named and unnamed arguments, consecutive named arguments form separate groups. So e.g. foo : (int, ~x:int, ~y:int, int, ~z:int) => int is exported to a JS function of type (number, {x:number, y:number}, number, {z:number}) => number.

Function components are exported and imported exactly like normal functions. For example:

[@genType]
[@react.component]
let make = (~name) => React.string(name);

For renaming, named arguments follow bucklescript's mangling convention:

Remove trailing "__" if present.
Otherwise remove leading "_" when followed by an uppercase letter, or keyword.

For example:

[@genType]
let exampleFunction = (~_type) => "type: " ++ _type;

record components

ReasonReact record components with props of Reason types t1, t2, t3 are exported as reactjs components with props of the JS types corresponding to t1, t2, t3. The annotation is on the make function: [@genType] let make ....

A file can export many components by defining them in sub-modules. The toplevel component is also exported as default.

imported types

It's possible to import an existing TS/Flow type as an opaque type in Reason. For example,

[@genType.import "./SomeFlowTypes"] type weekday;

defines a type which maps to weekday in SomeFlowTypes.js.
See for example Types.re and SomeFlowTypes.js.

recursive types

Recursive types which do not require a conversion are fully supported.
If a recursive type requires a conversion, only a shallow conversion is performed, and a warning comment is included in the output. (The alternative would be to perform an expensive conversion down a data structure of arbitrary size).
See for example Types.re.

first class modules

Reason first class modules are converted from their array Reason runtime representation to JS Object types.
For example,

module type MT = { let x: int; let y: string; };
module M = { let y = "abc"; let x = 42; };
[@genType] let firstClassModule: module MT = (module M);

is exported as a JS object of type

{x: number, y: string}

Notice how the order of elements in the exported JS object is determined by the module type MT and not the module implementation M.

polymorphic types

If a Reason type contains a type variable, the corresponding value is not converted. In other words, the conversion is the identity function. For example, a Reason function of type {payload: 'a} => 'a must treat the value of the payload as a black box, as a consequence of parametric polymorphism. If a typed back-end is used, the reason type is converted to the corresponding generic type.

exporting values from polymorphic types with hidden type variables

For cases when a value that contains a hidden type variable needs to be converted, a function can be used to produce the appropriate output:

Doesn't work

[@genType]
let none = None;

export const none: ?T1 = OptionBS.none; // Errors out as T1 is not defined

Works

[@genType]
let none = () => None;

const none = <T1>(a: T1): ?T1 => OptionBS.none;

promises

Valiues of type Js.Promise.t(arg) are exported to JS promises of type Promise<argJS> where argJS is the JS type corresponding to arg.
If a conversion for the argument is required, the conversion functions are chained via .then(promise => ...).

Experimental features

These features are for experimentation only. They could be changed/removed any time, and not be considered breaking changes.

• Export object and record types as interfaces. To activate, add "exportInterfaces": true to the configuration. The types are also renamed from name to Iname.

• Emit prop types for the untyped back-end. To activate, add "propTypes": true and "language": "untyped" to the configuration.

Limitations

• BuckleScript in-source = true. Currently only supports bucklescript projects with in-source generation and .bs.js file suffix.

• Limited namespace support. Currently there's limited namespace support, and only namespace:true is possible, not e.g. namespace:"custom".

Development/Contributing

Please check out our development instructions.