This is an example of the lengths to which pattern matching can be taken
in Rust. There's an enum variant named Declaration_ which contains a
value of type Declaration. The code you quoted is equivalent to the
following:
Declaration_(decl) => {
let l = decl.location;
let n = decl.name;
let v = decl.value;
let i = decl.important;
}
You can find out more about all of these types in the public Servo
documentation: http://doc.servo.org/cssparser/ast/struct.Declaration.html
The Rust guide also has a section on patterns that covers this usage:
http://doc.rust-lang.org/guide.html#patterns
Cheers,
Josh
On 2014-10-24 8:19 PM, Gilles Leblanc wrote:
Hello,
I'm having some problems understanding the following line of code in the
servo codebase:
https://github.com/servo/servo/blob/master/components/style/properties/mod.rs.mako#L1524
Declaration_(Declaration{ location: l, name: n, value: v, important: i}) =>
{
The code is doing some pattern matching. `_` normally matches against every
other case. In this case it's preceded by Declaration which I understand as
if `item` matches the result of any function starting with Declaration and
with any suffix which takes a parameter of type Declaration which is
probably a struct in which we destructure the values localtion, name, value
and important.
I can't seem to find where Declaration is defined, where `mportant` comes
from and I'm really not sure about my interpretation of the meaning of this
line of code.
Any pointers would be appreciated,
Thanks.
Here's the line in context:
match item {
DeclAtRule(rule) => log_css_error(
rule.location, format!("Unsupported at-rule in
declaration list: @{:s}", rule.name).as_slice()),
Declaration_(Declaration{ location: l, name: n, value: v,
important: i}) => {
// TODO: only keep the last valid declaration for a given name.
let (list, seen) = if i {
(&mut important_declarations, &mut important_seen)
} else {
(&mut normal_declarations, &mut normal_seen)
};
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