The exception handling question mark, spelled ? and abbreviated and pronounced eh?, is a half-arsed copy of monadic error handling. Rust devs really wanted the syntax without introducing HKTs, and admittedly you can't do foo()?.bar()?.baz()? in Haskell so it's only theoretical purity which is half-arsed, not ergonomics.

[–] Nevoic@lemmy.world 1 points 1 year ago* (last edited 1 year ago) (1 children)

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 &lt;&amp;> 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
    (?.) = (&lt;&amp;>)

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
    (?.) = (&lt;&amp;>)

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

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

[–] barsoap@lemm.ee 1 points 1 year ago* (last edited 1 year ago)

I was thinking along the lines of "you can't easily get at the wrapped type". To get at b instead of Maybe b you need to either use do-notation or lambdas (which do-notation is supposed to eliminate because they're awkward in a monadic context) whereas Rust will gladly hand you that b in the middle of an expression, and doesn't force you to name the point.

Or to give a concrete example, if foo()? {...} is rather awkward in Haskell, you end up writing things like

foo x y = bar >>= baz x y
  where
    baz x y True = x
    baz x y False = y

, though of course baz is completely generic and can be factored out. I think I called it "cap" in my Haskell days, for "consequent-alternative-predicate".

Flattening Functors and Monads syntax-wise is neat but it's not getting you all the way. But it's the Haskell way: Instead of macros, use tons upon tons of trivial functions :)

[–] m_f@midwest.social 1 points 1 year ago (1 children)

It's not a half-arsed copy, it's borrowing a limited subset of HKT for a language with very different goals. Haskell can afford a lot of luxuries that Rust can't.

[–] barsoap@lemm.ee 1 points 1 year ago

It's a specialised syntax transformation that has nothing to do with HKTs, or the type system in general. Also HKTs aren't off the table it's just that their theory isn't exactly trivial in face of the rest of Rust's type system but we already have GATs.

It actually wouldn't be hard writing a macro implementing do-notation that desugars to and_then calls on a particular type to get some kind of generic code (though of course monomorphised), but of course that would be circumventing the type system.

Anyhow my point stands that how Rust currently does it is imitating all that Haskell goodness on a practical everyday coding level but without having (yet) to solve the hard problem of how to do it without special-cased syntax sugar. With proper monads we e.g. wouldn't need to have separate syntax for async and ?

[–] arc@lemm.ee 0 points 1 year ago

You can say it's half-arsed if you like, but it's still vastly more convenient to write than if err != nil all over the place