CODEWITHSUNDEEP
typescripteitherfp-ts
David Novák
David Novák

Reputation: 1495

Using fp-ts sequenceS on Eithers with different left types

I want to use sequenceS function but it does not correctly infer left types. Let's consider the following code:

declare function validateEmail(
  email: string
): E.Either<EmailValidationError, Email>
declare function validateUsername(
  username: string
): E.Either<UsernameValidationError, Email>

function validateData(inputs: {
  email: string
  username: string
}) {
  return sequenceS(E.Apply)({
    email: validateEmail(inputs.email),
    username: validateUsername(inputs.username),
  })
}

I get a following error: Type 'UsernameValidationError' is not assignable to type 'EmailValidationError'.

I know I can fix it by specifying the types explicitly like so:

function validateData(inputs: {email: string; username: string}) {
  return sequenceS(E.Apply as apply.Apply2C<
    'Either', UsernameValidationError | EmailValidationError
  >)({
    email: validateEmail(inputs.email),
    username: validateUsername(inputs.username),
  })
}

But that does not seem practical especially with big objects created this way. I can also use dot notation but that still leaves me wandering if I am doing something wrong with this sequenceS use. Is there a way how to tell typescript to automatically infer the left types?

Upvotes: 3

Views: 204

Answers (1)

Denis Frezzato
Denis Frezzato

Reputation: 968

For how sequenceS is defined it expects values with fixed types on Left. For your use case you can use Do notation.

function validateData(inputs: {
  email: string
  username: string
}) {
  return pipe(
    validateEmail(inputs.email),
    E.bindTo('email'),
    E.apSW('username', validateUsername(inputs.username),
  )
}

Upvotes: 0

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