this post was submitted on 14 Aug 2023
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Swift and Rust have a far more elegant solution. Swift has a pseudo throw / try-catch, while Rust has a Result<> and if you want to throw it up the chain you can use a ? notation instead of cluttering the code with error checking.
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 returnsB
you'd end up with
B?
in either caseSay you have these functions
and you want to construct the following using these 3 functions
in a Rust-type syntax, you'd call
in Haskell you'd have two different operators here
however you can define this type class
and then get roughly the same syntax as rust without introducing a new language feature
though this is more general than just
Maybe
s (it works with any functor/monad), and maybe you wouldn't want it to be. In that case you'd do thisrestricting 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 ofMaybe 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 thatb
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, 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?
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