An example of using the Either type

using-either-type.ts

import * as E from 'fp-ts/Either';

// 1.

const MY_VAR_UNSAFE: string | undefined = process.env['MY_VAR'];

const MY_VAR_NOT_DEFINED = 'Environment variable MY_VAR not set.' as const;

const MY_VAR_EITHER: E.Either<typeof MY_VAR_NOT_DEFINED, string> =
  MY_VAR_UNSAFE !== undefined
    ? E.right(MY_VAR_UNSAFE)
    : E.left(MY_VAR_NOT_DEFINED);

const main = () => {
  if (E.isLeft(MY_VAR_EITHER)) {
    console.error(MY_VAR_EITHER.left);
    return;
  }
  console.log(MY_VAR_EITHER.right);
};

main();

// 2.

export const div = (
  a: number,
  b: number
): E.Either<'Division by zero.', number> => {
  if (b === 0) {
    return E.left('Division by zero.' as const);
  }
  return E.right(a / b);
};