What are GADTs?

GADTs: A primer

Install

Install the last stable version

For BuckleScript < 6

npm install --save-dev [email protected]
or
yarn add [email protected] -D

For BuckleScript >= 6

npm install --save-dev [email protected]
or
yarn add [email protected] -D

Build

npm run build

Watch

npm run watch

In

module StateLenses = [%lenses
 type state = {
   email: string,
   age: int,
 }
]

Out

module StateLenses = {
  type state = {
    email: string,
    age: int,
  };
  type field(_) =
    | Email: field(string)
    | Age: field(int);
  let get: type value. (state, field(value)) => value =
    (state, field) =>
      switch (field) {
      | Email => state.email
      | Age => state.age
      };
  let set: type value. (state, field(value), value) => state =
    (state, field, value) =>
      switch (field) {
      | Email => {...state, email: value}
      | Age => {...state, age: value}
      };
};

Using

open StateLenses;

let state = {email: "[email protected]", age: 969};

Js.log(state->get(Email));
Js.log(state->get(Age));

Alternatively you can also use it like

[@lenses]
[@decco]
type bartux = {
  color: string,
  top: int,
};

let bartux = {color: "red", top: 10};

Js.log(bartux->bartux_get(Color));
Js.log(bartux->bartux_set(Top, 20));
Js.log(bartux_encode(bartux));