How to branch pipe

[simPod]

Consider I want to validate value. Valid value is either empty OR run through validation chain. I know how to validate the value using the chain:

(from https://dev.to/gcanti/getting-started-with-fp-ts-either-vs-validation-5eja)

import { Either, left, right } from 'fp-ts/Either'

const minLength = (s: string): Either<string, string> =>
  s.length >= 6 ? right(s) : left('at least 6 characters')

const oneCapital = (s: string): Either<string, string> =>
  /[A-Z]/g.test(s) ? right(s) : left('at least one capital letter')

const oneNumber = (s: string): Either<string, string> =>
  /[0-9]/g.test(s) ? right(s) : left('at least one number')


import { chain } from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

const validatePassword = (s: string): Either<string, string> =>
  pipe(
    minLength(s),
    chain(oneCapital),
    chain(oneNumber)
  )

Then I do

pipe(
  value,
  valiDatePassword
)

However, I'd like to run valiDatePassword only if the value is not empty string ''.

Basically trying to write this in FP:

const validatePassword = (value: string): bool => ...;

if (value === '') {
  return true;
}

return validatePassword(value);

Thanks!

[samhh]

I was thinking something like this:

foldMap validatePassword . guarded (not . null)

It doesn't translate nearly as cleanly though:

flow(
  O.fromPredicate(not(Str.isEmpty)),
  O.foldMap(getApplicativeMonoid(E.Applicative)<string, string>(Str.Monoid))(validatePassword),
)

You could always just do:

x => x === '' ? E.of('') : validatePassword(x),

Edit: @gcanti's is better!

[simPod]

Thanks anyway!

[gcanti]

chain let you combine parsers under an AND condition, alt let you combine parsers under an OR condition, you could define a parser for empty strings (which could be possibly reused in other circumstances):

import { fromPredicate, alt } from 'fp-ts/Either'
import { isEmpty } from 'fp-ts/string'

const empty = fromPredicate(isEmpty, () => 'non empty string')

const program = (s: string): Either<string, string> =>
  pipe(
    empty(s),
    alt(() => validatePassword(s))
  )

pipe('', program, console.log) // { _tag: 'Right', right: '' }
pipe('aaa', program, console.log) // { _tag: 'Left', left: 'at least 6 characters' }

[simPod]

Works like a charm, thanks for pointers!