Here’s an example (first in Haskell then in Go), lets say you have some types/functions:

• type Possible a = Either String a
• data User = User { name :: String, age :: Int }
• validateName :: String -> Possible String
• validateAge :: Int -> Possible Int

then you can make:

mkValidUser :: String -> Int -> Possible User
mkValidUser name age = do
  validatedName <- validateName name
  validatedAge  <- validateAge age
  pure $ User validatedName validatedAge

in Go you’d have these

• (no Possible type alias, Go can’t do generic type aliases yet, there’s an open issue for it)
• type User struct { Name string; Age int }
• func validateName(name string) (string, error)
• func validateAge(age int) (int, error)

and with them you’d make:

func mkValidUser(name string, age int) (*User, error) {
  validatedName, err = validateName(name)
  if err != nil {
    return err
  }

  validatedAge, err = validateAge(age)
  if err != nil {
    return err
  }

  return User(Name: validatedName, Age: validatedAge)
}

In the Haskell, the fact that Either is a monad is saving you from a lot of boilerplate. You don’t have to explicitly handle the Left/error case, if any of the Eithers end up being a Left value then it’ll correctly “short-circuit” and the function will evaluate to that Left value.

Without using the fact that it’s a functor/monad (e.g you have no access to fmap/>>=/do syntax), you’d end up with code that has a similar amount of boilerplate to the Go code (notice we have to handle each Left case now):

mkValidUser :: String -> Int -> Possible User
mkValidUser name age =
  case (validatedName name, validateAge age) of
    (Left nameErr, _) => Left nameErr
    (_, Left ageErr)  => Left ageErr
    (Right validatedName, Right validatedAge) => 
      Right $ User validatedName validatedAge