Note: Lemmy code blocks don’t play nice with some symbols, specifically < and & in the following code examples

This isn’t a language level issue really though, Haskell can be equally ergonomic.

The weird thing about ?. is that it’s actually overloaded, it can mean:

• call a function on A? that returns B?
• call a function on A? that returns B

you’d end up with B? in either case

Say you have these functions

toInt :: String -> Maybe Int

double :: Int -> Int

isValid :: Int -> Maybe Int

and you want to construct the following using these 3 functions

fn :: Maybe String -> Maybe Int

in a Rust-type syntax, you’d call

str?.toInt()?.double()?.isValid()

in Haskell you’d have two different operators here

str >>= toInt <&> double >>= isValid

however you can define this type class

class Chainable f a b fb where
    (?.) :: f a -> (a -> fb) -> f b

instance Functor f => Chainable f a b b where
    (?.) = (<&>)

instance Monad m => Chainable m a b (m b) where
    (?.) = (>>=)

and then get roughly the same syntax as rust without introducing a new language feature

str ?. toInt ?. double ?. isValid

though this is more general than just Maybes (it works with any functor/monad), and maybe you wouldn’t want it to be. In that case you’d do this

class Chainable a b fb where
    (?.) :: Maybe a -> (a -> fb) -> Maybe b

instance Chainable a b b where
    (?.) = (<&>)

instance Chainable a b (Maybe b) where
    (?.) = (>>=)

restricting it to only maybes could also theoretically help type inference.

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

Reddit is an interest of mine, but I have no interest in going to reddit anymore. Is that an unimaginable position for you?

No, they use ActivityPub underneath, so you can go to kbin/mastodon communities/users/etc. from Lemmy and vice versa.