Hacker News Reader

Comment by noelwelsh | Hacker News Reader

As some of the Rust developers hang around here, a question:

Result is the Either monad in Scala / Haskell? So IO functions return either a success wrapping a value, or a failure? This is great -- I'm a big fan of this style of error handling. However it seems that Rust doesn't have syntax for monads -- the equivalent of Haskell's do and Scala's for. This makes monads a bit of a PITA. Is there a plan to address this?

View on HN

People keep wanting monadic syntax (I'm one of them), but I believe the general suggestion is to do it via macros/syntax extensions. E.g. write something that desugars

   do! {
       bind x = foo();
       let bar = baz(10);
       ret x + bar
   }

into the equivalent of Haskell's

   do 
    x <- foo
    let bar = baz 10
    return $ x + bar

Rust currently lacks higher-kinded types, so it would be hard to write functions that are generic over monads but the above would at least give some local reprieve.

Also, note the `if_ok!` macro alluded to in the point about IO errors is giving the shortcircuiting behaviour of the Either monad (specific to Either, though).

(NB. the macro probably can't be called `do` just yet, but we may remove that as a keyword. Maybe.)

Wait, how don't macro need to take escaped sigils?

Like range! takes range!('[' 0 , Inf ')' )?

Is 'do' still a keyword? I remember it being removed, pretty much everywhere. I just assumed it was thrown away.

> Wait, how don't macro need to take escaped sigils?

These are proper (hygienic) syntax macros, much like Scheme's syntax-rules. C's "macros" only work at the token level.

> Is 'do' still a keyword?

I believe it is still a reserved keyword, but not used anymore.

It appears to not be a reserved word at all:

  $ echo 'fn main() { let do = 1; }' | rustc -

runs with just an unused variable warning (if it were reserved it would fail with "error: `do` is a reserved keyword").

It probably should be reserved.

They need to take balanced braces/parens/brackets (i.e. only escaped if they are possibly unbalanced), so that Rust source can still be parsed without having to run the macro expander concurrently.

And, it looks like do is no longer a keyword at all, I must've missed that part of the do-removal patch! So it is possible to call that macro do.

As Rust ML lurker - there was some talk about using macros to fill in for this pain point, if I recall correctly. From what I've gathered do is dead and gone.

For anyone not familiar with it, the `do` that was removed provided sugar for calling HOFs similar to Ruby blocks.

Monadic do could be added in the future, backwards compatibly.

"if_ok!" is designed to handle this. It's basically the equivalent of monadic do, but works better with the control flow structures of Rust.

Is there a location in the docs that lists and describes the built-in macros in Rust? They don't appear to be findable using the stdlib search, and the Macros tutorial doesn't seem to talk about built-ins.

Also, this post forced me to read the docs a bit more thoroughly and realize that I haven't been distinguishing between Option and Result. Thanks

Reply on news.ycombinator.com