5 ms·

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 d

by dbaupp 13y ago

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.)

Ygg2 13y ago
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.
- lucian1900 13y ago
  > 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.
  - dbaupp 13y ago
    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").
    - brson 13y ago
      It probably should be reserved.
- dbaupp 13y ago
  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.

by dbaupp 13y ago

Ygg2 13y ago

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.

lucian1900 13y ago
> 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.
- dbaupp 13y ago
  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").
  - brson 13y ago
    It probably should be reserved.
dbaupp 13y ago
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.