On 24/01/14 15:32, benjamin adamson wrote:
Question, what constitutes whether a 'trait' is applicable for
implementation by the #deriving() attribute?
According to the language specification on master:
http://static.rust-lang.org/doc/master/rust.html#deriving
There exists a static list. I found myself interested in the idea of
using the deriving attribute to derive a simple drawable implementation:
https://github.com/JeremyLetang/rust-sfml/blob/master/src/rsfml/traits/drawable.rs
but then looked up the attribute in the rust manual, and noticed that
there is a static list of what I will call 'traits that support the
deriving attribute'. Why the restriction? Is there some prior reading
on this? Is there any plan on letting libraries define more types that
can be 'derivable'?
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There's a static list because they're currently hard-coded into the
compiler.
With external syntax extensions, it's now possible to define something
like `#[deriving_Drawable]`, and, iirc, I left the core deriving
infrastructure public in syntax::ext::deriving::generic, so you can get
most of the work done for you that way.
There are various things to be worked out for adding traits directly to
#[deriving(Foo)], including (but not limited to) how namespacing works
(e.g. if two libraries both define a trait with the same name and
provide #[deriving] implementations for it), and whether we want it to
be a priviledged syntax extension where users can directly add to its
map between traits and deriving implementation.
Huon
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