Macro select 

macro_rules! select {
    ($($module:ident)::+ : + $field:ident $($rest:tt)*) => { ... };
    ($($module:ident)::+ : - $field:ident $($rest:tt)*) => { ... };
    ($($module:ident)::+ :) => { ... };
    ($($module:ident)::+ : $field:ident) => { ... };
    ($($module:ident)::+ : $field:ident,) => { ... };
    ($($module:ident)::+ : ? $field:ident) => { ... };
    ($($module:ident)::+ : ? $field:ident,) => { ... };
    ($($module:ident)::+ : $field:ident, $($rest:tt)+) => { ... };
    ($($module:ident)::+ : ? $field:ident, $($rest:tt)+) => { ... };
}

Construct a FieldSet by selecting fields from a module.

Syntax

• select!(module: field) - field is Present
• select!(module: ?field) - field is Optional
• select!(module: ?all) - all fields Optional
• Multiple fields compose: select!(module: field1, ?field2) expands to module::with::field1::Present<module::with::field2::Optional>

Examples

type Create = select!(user: name, email);
type Update = select!(user: ?name, ?email);
fn create_user(user: User<Create>) {...}