how to use pipe in fp-ts to remove if statement

Question

I am having a look at pipe and Option in fp-ts for the first time.

I have this code which does type narrow but I'm sure it can be done without the if statement:

if (O.isNone(this.state)) {
  return undefined;
}

return this.lens.get(this.state.value);

Answer 1

You can try to use pipe in combination with Option.fromNullable and Option.map:

import { pipe } from "fp-ts/function";
import * as O from "fp-ts/Option";

let obj = {
  state: {
    value: "test"
  }
};

function calculate(input: { value: string }) {
  return input.value;
}

console.log(
  pipe(
    obj.state,
    O.fromNullable,
    O.map((value) => calculate(value))
  )
);

So for your example, it would be like:

return pipe(
  this.state,
  O.fromNullable,
  O.map(state => this.lens.get(state.value))
);

Answer 2

Usually, the idea with all those wrapping data types is that you don't want to unwrap too early. In your case, considering that this.state is an Option, I would do this:

import { option } from 'fp-ts';

pipe(
  this.state,
  option.map(state => this.lens.get(state)),
  option.toUndefined,
);