Unable to convert from Result<T,E> to Option<T>

[lbonn]

I tried to use the existing conversion provided by the ToOption<T> trait which Result<T,E> implements:

#[deriving(Clone)]
struct A;

fn main() {
    let a : Result<A,int> = Ok(A);
    a.to_option();
}

But apparently, the compiler does not find its implementation:

option.rs:6:4: 6:18 error: type `std::result::Result<A,int>` does not implement any method in scope named `to_option`
option.rs:6     a.to_option();
                ^~~~~~~~~~~~~~
error: aborting due to previous error

Maybe I've done something wrong but I'm rather dry on ideas right now...

[sfackler]

You need to add a use std::option::ToOption; to the top of the file. Methods defined by a trait are only callable if they're imported (some traits are implicitly imported in the prelude).

[alexcrichton]

@sfackler is correct, this is working as intended (plus his explanation is spot on)

[lbonn]

Ok I feel a bit dumb :), a re-read of the doc on traits and modules may be worth it.

On the other hand, it may be nice to have a visual help in the libstd docs to immediately see if a type/trait is re-exported by the prelude (or by another module maybe?). I'll try to look into rustdoc to see if this is doable.