What are GADTs?

GADTs: A primer

Why

Differently from normal lenses/optics the following approach allows for composing "lenses" into lists/arrays which is them useful for things like https://github.com/rescriptbr/reschema

Install

Install the last stable version

For ReScript

npm install --save-dev lenses-ppx@latest
or
yarn add lenses-ppx@latest -D

For BuckleScript < 6

npm install --save-dev lenses-ppx@4.0.0
or
yarn add lenses-ppx@4.0.0 -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 rec 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: "fakenickels@gov.br", 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))

Alternatives

• https://github.com/scoville/re-optic/blob/master/docs/lenses-ppx.md which is more strict and follows more closely Optics standards