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.
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.
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 :)
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.
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 ?
The exception handling question mark, spelled
?and abbreviated and pronouncedeh?, is a half-arsed copy of monadic error handling. Rust devs really wanted the syntax without introducing HKTs, and admittedly you can’t dofoo()?.bar()?.baz()?in Haskell so it’s only theoretical purity which is half-arsed, not ergonomics.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:A?that returnsB?A?that returnsByou’d end up with
B?in either caseSay you have these functions
toInt :: String -> Maybe Int double :: Int -> Int isValid :: Int -> Maybe Intand you want to construct the following using these 3 functions
fn :: Maybe String -> Maybe Intin a Rust-type syntax, you’d call
str?.toInt()?.double()?.isValid()in Haskell you’d have two different operators here
str >>= toInt <&> double >>= isValidhowever 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 ?. isValidthough 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 thisclass 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.
I was thinking along the lines of “you can’t easily get at the wrapped type”. To get at
binstead ofMaybe byou 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 thatbin 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 likefoo 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 :)
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.
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_thencalls 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
asyncand?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