I am trying to implement a function with an optional reference. I would like to be able to use it like this:
fn main() {
let x = 557483943;
test_function(x, None);
let params = parameters(x);
test_function(x, Some(¶ms));
}
where parameters(x) may take some time to execute. Therefore I would like to be able to pass an optional reference to the function. For now I use this as test code:
#[derive(Debug)]
pub struct Parameters {
a: u32,
b: u32,
}
pub fn parameters(x: u32) -> Parameters {
// would be a tedious calculation...
let a = x / 6453;
let b = x % 589703;
Parameters { a, b }
}
What I came up with is this:
pub fn test_function(x: u32, params: Option<&Parameters>) {
// tmp_params is needed as owner of the return value of
// parameters(x) that outlives the match statement
let tmp_params: Parameters;
let params = match params {
Some(params) => params,
None => {
tmp_params = parameters(x);
&tmp_params
},
};
println!("params: {:?}", ¶ms);
}
which works, but I am not sure if this is the idiomatic way to do this in rust.
Is there a way to avoid tmp_params and have the Parameters created in the match statement outlive the match statement itself?
Is there a better/more idiomatic way to do this?
Related
I'm making my own Serializable trait, in the context of a client / server system.
My idea was that the messages sent by the system is an enum made by the user of this system, so it can be customize as needed.
Too ease implementing the trait on the enum, I would like to use the #[derive(Serializable)] method, as implementing it is always the same thing.
Here is the trait :
pub trait NetworkSerializable {
fn id(&self) -> usize;
fn size(&self) -> usize;
fn serialize(self) -> Vec<u8>;
fn deserialize(id: usize, data: Vec<u8>) -> Self;
}
Now, I've tried to look at the book (this one too) and this example to try to wrap my head around derive macros, but I'm really struggling to understand them and how to implement them. I've read about token streams and abstract trees, and I think I understand the basics.
Let's take the example of the id() method : it should gives a unique id for each variant of the enum, to allow headers of messages to tell which message is incoming.
let's say I have this enum as a message system :
enum NetworkMessages {
ErrorMessage,
SpawnPlayer(usize, bool, Transform), // player id, is_mine, position
MovePlayer(usize, Transform), // player id, new_position
DestroyPlayer(usize) // player_id
}
Then, the id() function should look like this :
fn id(&self) -> usize {
match &self {
&ErrorMessage => 0,
&SpawnPlayer => 1,
&MovePlayer => 2,
&DestroyPlayer => 3,
}
}
Here was my go with writting this using a derive macro :
#[proc_macro_derive(NetworkSerializable)]
pub fn network_serializable_derive(input: TokenStream) -> TokenStream {
// Construct a representation of Rust code as a syntax tree
// that we can manipulate
let ast = syn::parse(input).unwrap();
// Build the trait implementation
impl_network_serializable_macro(&ast)
}
fn impl_network_serializable_macro(ast: &syn::DeriveInput) -> TokenStream {
// get enum name
let ref name = ast.ident;
let ref data = ast.data;
let (id_func, size_func, serialize_func, deserialize_func) = match data {
// Only if data is an enum, we do parsing
Data::Enum(data_enum) => {
// Iterate over enum variants
let mut id_func_internal = TokenStream2::new();
let mut variant_id: usize = 0;
for variant in &data_enum.variants {
// add the branch for the variant
id_func_internal.extend(quote_spanned!{
variant.span() => &variant_id,
});
variant_id += 1;
}
(id_func_internal, (), (), ())
}
_ => {(TokenStream2::new(), (), (), ())},
};
let expanded = quote! {
impl NetworkSerializable for #name {
// variant_checker_functions gets replaced by all the functions
// that were constructed above
fn size(&self) -> usize {
match &self {
#id_func
}
}
/*
#size_func
#serialize_func
#deserialize_func
*/
}
};
expanded.into()
}
So this is generating quite a lot of errors, with the "proc macro NetworkSerializable not expanded: no proc macro dylib present" being first. So I'm guessing there a lot of misunderstaning from my part in here.
Using an async predicate to filter a list of values makes Rust complain about lifetimes. Even if the collection is awaited, which means the predicate will not outlive the filtered value, Rust remains skeptical.
Full repro below with playground here. Note that it filters on a non-copy struct we'd rather pass by reference, rather than a simple value we could just copy and forget without incurring in overhead.
use futures::stream::iter;
use futures::StreamExt;
#[derive(Debug)]
struct Foo {
bar: usize
}
impl Foo {
fn new(bar: usize) -> Self {
Self {
bar
}
}
}
#[tokio::main]
async fn main() {
let arr = vec![
Foo::new(0),
Foo::new(1),
Foo::new(2)
];
let filtered = iter(arr)
.filter(|f| async {compute_baz(f).await > 0})
.collect::<Vec<_>>()
.await;
// should print Foo{bar:1} and Foo{bar:2}
println!("{:?}", filtered)
}
async fn compute_baz(foo: &Foo) -> usize {
// ...do lengthy task...
foo.bar
}
Update
As #Ceasar pointed out below, the async functions are not run in parallel, can that be done?
I'm trying to do something like:
let filter_mask = join_all(items.map(predicate));
let filtered = items.filter(|i| filter_mask[i]).collect::<Vec<_>>();
without the clutter.
An easy workaround is to avoid the closure:
let mut filtered = vec![];
for f in arr.iter() {
if compute_baz(f).await > 0 {
filtered.push(f);
}
}
I've been trying to implement a Strategy pattern in rust, but I'm having trouble understanding how to make it work.
So let's imagine we have a trait Adder and Element:
pub trait Element {
fn to_string(&self) -> String;
}
pub trait Adder {
type E: Element;
fn add (&self, a: &Self::E, b: &Self::E) -> Self::E;
}
And we have two implementations StringAdder with StringElements and UsizeAdder with UsizeElements:
// usize
pub struct UsizeElement {
pub value: usize
}
impl Element for UsizeElement {
fn to_string(&self) -> String {
self.value.to_string()
}
}
pub struct UsizeAdder {
}
impl Adder for UsizeAdder{
type E = UsizeElement;
fn add(&self, a: &UsizeElement, b: &UsizeElement) -> UsizeElement{
UsizeElement { value: a.value + b.value }
}
}
// String
pub struct StringElement {
pub value: String
}
impl Element for StringElement {
fn to_string(&self) -> String {
self.value.to_string()
}
}
pub struct StringAdder {
}
impl Adder for StringAdder {
type E = StringElement;
fn add(&self, a: &StringElement, b: &StringElement) -> StringElement {
let a: usize = a.value.parse().unwrap();
let b: usize = b.value.parse().unwrap();
StringElement {
value: (a + b).to_string()
}
}
}
And I want to write a code that uses trait methods from Adder trait and it's corresponding elements without knowing at compile time which strategy is going to be used.
fn main() {
let policy = "usize";
let element = "1";
let adder = get_adder(&policy);
let element_a = get_element(&policy, element);
let result = adder.add(element_a, element_a);
}
To simplify I'm going to assign a string to policy and element but normally that would be read from a file.
Is the only way to implement get_adder and get_element using dynamic dispatch? And by extension should I define Adder and Element traits to use trait objects and or the Any trait?
Edit: Here is what I managed to figure out so far.
An example of possible implementation is using match to help define concrete types for the compiler.
fn main() {
let policy = "string";
let element = "1";
let secret_key = "5";
let result = cesar(policy, element, secret_key);
dbg!(result.to_string());
}
fn cesar(policy: &str, element: &str, secret_key: &str) -> Box<dyn Element>{
match policy {
"usize" => {
let adder = UsizeAdder{};
let element = UsizeElement{ value: element.parse().unwrap() };
let secret_key = UsizeElement{ value: secret_key.parse().unwrap() };
Box::new(cesar_impl(&adder, &element, &secret_key))
}
"string" => {
let adder = StringAdder{};
let element = StringElement{ value: element.to_string() };
let secret_key = StringElement{ value: secret_key.to_string() };
Box::new(cesar_impl(&adder, &element, &secret_key))
}
_ => {
panic!("Policy not supported!")
}
}
}
fn cesar_impl<A>(adder: &A, element: &A::E, secret_key: &A::E) -> A::E where A: Adder, A::E : Element {
adder.add(&element, &secret_key)
}
However the issue is that I have to wrap every function I want to implement using a match function to determine the concrete type, and also case for every policy available.
It does not seem like the proper way of implementing it as it will bloat the code, make it more error prone and less maintainable unless I end up using macros.
Edit 2: Here you can find an example using dynamic dispatch. However I'm not convinced it's the proper way to implement the solution.
Example using dynamic dispatch
Thank you for your help :)
Edit
As it seemms from the suggested solution, What I'm trying to achieve seems impossible/Not the correct way, therefore - I'll explain the end goal here:
I am parsing the values for Foo from a YAML file using serde, and I would like to let the user get one of those stored values from the yaml at a time, this is why I wanted to store an iterator in my struct
I have two struct similar to the following:
struct Bar {
name: String,
id: u32
}
struct Foo {
my_map: HashMap<String, Bar>
}
In my Foo struct, I wish to store an iterator to my HashMap, so a user can borrow values from my map on demand.
Theoretically, the full Foo class would look something like:
struct Foo {
my_map: HashMap<String, Bar>,
my_map_iter: HashMap<String, Bar>::iterator
}
impl Foo {
fn get_pair(&self) -> Option<(String, Bar)> {
// impl...
}
}
But I can't seem to pull it off and create such a variable, no matter what I try (Various compilation errors which seems like I'm just trying to do that wrong).
I would be glad if someone can point me to the correct way to achieve that and if there is a better way to achieve what I'm trying to do - I would like to know that.
Thank you!
I am parsing the values for Foo from a YAML file using serde
When you parse them you should put the values in a Vec instead of a HashMap.
I imagine the values you have also have names which is why you thought a HashMap would be good. You could instead store them like so:
let parsed = vec![]
for _ in 0..n_to_parse {
// first item of the tuple is the name second is the value
let key_value = ("Get from", "serde");
parsed.push(key_value);
}
then once you stored it like so it will be easy to get the pairs from it by keeping track of the current index:
struct ParsedHolder {
parsed: Vec<(String, String)>,
current_idx: usize,
}
impl ParsedHolder {
fn new(parsed: Vec<(String, String)>) -> Self {
ParsedHolder {
parsed,
current_idx: 0,
}
}
fn get_pair(&mut self) -> Option<&(String, String)> {
if let Some(pair) = self.parsed.get(self.current_idx) {
self.current_idx += 1;
Some(pair)
} else {
self.current_idx = 0;
None
}
}
}
Now this could be further improved upon by using VecDeque which will allow you to efficiently take out the first element of parsed. Which will make it easy to not use clone. But this way you will be only able to go through all the parsed values once which I think is actually what you want in your use case.
But I'll let you implement VecDeque 😃
The reason why this is a hard is that unless we make sure the HashMap isn't mutated while we iterate we could get into some trouble. To make sure the HashMap is immutable until the iterator lives:
use std::collections::HashMap;
use std::collections::hash_map::Iter;
struct Foo<'a> {
my_map: &'a HashMap<u8, u8>,
iterator: Iter<'a, u8, u8>,
}
fn main() {
let my_map = HashMap::new();
let iterator = my_map.iter();
let f = Foo {
my_map: &my_map,
iterator: iterator,
};
}
If you can make sure or know that the HashMap won't have new keys or keys removed from it (editing values with existing keys is fine) then you can do this:
struct Foo {
my_map: HashMap<String, String>,
current_idx: usize,
}
impl Foo {
fn new(my_map: HashMap<String, String>) -> Self {
Foo {
my_map,
current_idx: 0,
}
}
fn get_pair(&mut self) -> Option<(&String, &String)> {
if let Some(pair) = self.my_map.iter().skip(self.current_idx).next() {
self.current_idx += 1;
Some(pair)
} else {
self.current_idx = 0;
None
}
}
fn get_pair_cloned(&mut self) -> Option<(String, String)> {
if let Some(pair) = self.my_map.iter().skip(self.current_idx).next() {
self.current_idx += 1;
Some((pair.0.clone(), pair.1.clone()))
} else {
self.current_idx = 0;
None
}
}
}
This is fairly inefficient though because we need to iterate though the keys to find the next key each time.
Out of habit from interpreted programming languages, I want to rewrite many values based on their key. I assumed that I would store all the information in the struct prepared for this project. So I started iterating:
struct Container {
x: String,
y: String,
z: String
}
impl Container {
// (...)
fn load_data(&self, data: &HashMap<String, String>) {
let valid_keys = vec_of_strings![ // It's simple vector with Strings
"x", "y", "z"
] ;
for key_name in &valid_keys {
if data.contains_key(key_name) {
self[key_name] = Some(data.get(key_name);
// It's invalid of course but
// I do not know how to write it correctly.
// For example, in PHP I would write it like this:
// $this[$key_name] = $data[$key_name];
}
}
}
// (...)
}
Maybe macros? I tried to use them. key_name is always interpreted as it is, I cannot get value of key_name instead.
How can I do this without repeating the code for each value?
With macros, I always advocate starting from the direct code, then seeing what duplication there is. In this case, we'd start with
fn load_data(&mut self, data: &HashMap<String, String>) {
if let Some(v) = data.get("x") {
self.x = v.clone();
}
if let Some(v) = data.get("y") {
self.y = v.clone();
}
if let Some(v) = data.get("z") {
self.z = v.clone();
}
}
Note the number of differences:
The struct must take &mut self.
It's inefficient to check if a value is there and then get it separately.
We need to clone the value because we only only have a reference.
We cannot store an Option in a String.
Once you have your code working, you can see how to abstract things. Always start by trying to use "lighter" abstractions (functions, traits, etc.). Only after exhausting that, I'd start bringing in macros. Let's start by using stringify
if let Some(v) = data.get(stringify!(x)) {
self.x = v.clone();
}
Then you can extract out a macro:
macro_rules! thing {
($this: ident, $data: ident, $($name: ident),+) => {
$(
if let Some(v) = $data.get(stringify!($name)) {
$this.$name = v.clone();
}
)+
};
}
impl Container {
fn load_data(&mut self, data: &HashMap<String, String>) {
thing!(self, data, x, y, z);
}
}
fn main() {
let mut c = Container::default();
let d: HashMap<_, _> = vec![("x".into(), "alpha".into())].into_iter().collect();
c.load_data(&d);
println!("{:?}", c);
}
Full disclosure: I don't think this is a good idea.