Someone else and not an expert. But Maybe types are implemented with Monads, Maybe is a common monad.

Its how rust does error handling for example, you have to test a return value for “something or nothing” but you can pass the monadic value and handle the error later, in go you have to handle the error explicitly (almost) all the time.

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

for some reason <- in lemmy shows up as <- inside code blocks, so I used the left arrow unicode in the above instead

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 nil, err
  }

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

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

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