I'm new to Rust, probably missing something obvious. I have the following code with the main idea of being able to index any struct field like so struct_instance['field'].
use std::ops::Index;
enum Selection {
Full,
Partial,
}
struct Config {
display: bool,
timeout: u16,
selection: Selection,
}
impl Index<&'_ str> for Config {
type Output = bool;
fn index(&self, index: &'_ str) -> &Self::Output {
match index {
"display" => &self.display,
_ => panic!("Unknown field: {}", index),
}
}
}
fn main() {
let config = Config {
display: true,
timeout: 500,
selection: Selection::Partial,
};
let display = config["display"];
println!("{display}");
}
The problem is: I can not find a way to index every type of struct fields, because associated type Output doesn't let me define more than one type. I would want to have match being able to process all Config fields somehow, is there a way to do so?
As answered apilat , Index is for array like structures.
However if you want, you can achieve this with enums.
Create enum with all available types of config fields (bool, u16, Selection, etc...)
Change Config fields' types to this new enum
Change the Output in the Index impl again to this new enum
Here is full code example
use std::ops::Index;
#[derive(Debug)]
enum ConfigField {
Display(bool),
Timeout(u16),
Selection(Selection)
}
#[derive(Debug)]
enum Selection {
Full,
Partial,
}
struct Config {
display: ConfigField,
timeout: ConfigField,
selection: ConfigField,
}
impl Index<&'_ str> for Config {
type Output = ConfigField;
fn index(&self, index: &'_ str) -> &Self::Output {
match index {
"display" => &self.display,
"timeout" => &self.timeout,
"selection" => &self.selection,
_ => panic!("Unknown field: {}", index),
}
}
}
fn main() {
let config = Config {
display: ConfigField::Display(true),
timeout: ConfigField::Timeout(500),
selection: ConfigField::Selection(Selection::Partial),
};
let display = &config["display"];
let timeout = &config["timeout"];
let selection = &config["selection"];
println!("{:?} {:?} {:?}", display, timeout, selection);
}
Related
Let's say I have a attrs: Vec<Attribute> of some function attributes and a function fn map_attribute(attr: &Attribute) -> Result<TokenStream, Error> that maps the attributes to some code.
I know that I could write something like this:
attrs.into_iter()
.map(map_attribute)
.collect::<Result<Vec<_>, _>()?
However, this is not what I want. What I want is spit out all errors at once, not stop with the first Error. Currently I do something like this:
let mut codes : Vec<TokenStream> = Vec::new();
let mut errors: Vec<Error> = Vec::new();
for attr in attrs {
match map_attribute(attr) {
Ok(code) => codes.push(code),
Err(err) => errors.push(err)
}
}
let mut error_iter = errors.into_iter();
if let Some(first) = error_iter.nth(0) {
return Err(iter.fold(first, |mut e0, e1| { e0.combine(e1); e0 }));
}
This second version does what I want, but is considerably more verbose than the first version. Is there a better / more idiomatic way to acchieve this, if possible without creating my own iterator?
The standard library does not have a convenient one-liner for this as far as I know, however the excellent itertools library does:
use itertools::Itertools; // 0.9.0
fn main() {
let foo = vec![Ok(42), Err(":("), Ok(321), Err("oh noes")];
let (codes, errors): (Vec<_>, Vec<_>)
= foo.into_iter().partition_map(From::from);
println!("codes={:?}", codes);
println!("errors={:?}", errors);
}
(Permalink to the playground)
I ended up writing my own extension for Iterator, which allows me to stop collecting codes when I encounter my first error. This is in my use case probably a bit more efficient than the answer by mcarton, since I only need the first partition bucket if the second one is empty. Also, I need to fold the errors anyways if I want to combine them into a single error.
pub trait CollectToResult
{
type Item;
fn collect_to_result(self) -> Result<Vec<Self::Item>, Error>;
}
impl<Item, I> CollectToResult for I
where
I : Iterator<Item = Result<Item, Error>>
{
type Item = Item;
fn collect_to_result(self) -> Result<Vec<Item>, Error>
{
self.fold(<Result<Vec<Item>, Error>>::Ok(Vec::new()), |res, code| {
match (code, res) {
(Ok(code), Ok(mut codes)) => { codes.push(code); Ok(codes) },
(Ok(_), Err(errors)) => Err(errors),
(Err(err), Ok(_)) => Err(err),
(Err(err), Err(mut errors)) => { errors.combine(err); Err(errors) }
}
})
}
}
I've written a custom protocol where I've defined my own struct for a frame and it parses from bytes. My function accepts a Vec and parses the elements accordingly. To account for invalid frames, I am returning a Result<Frame> and calling .get() on the byte array. Here's my code:
fn main(){
let emptyvec = Vec::new();
match Frame::from_bytes(emptyvec) {
Err(e) => {
println!("Received invalid frame");
},
Ok(frame) => {
println!("Received valid frame");
}
}
}
struct Frame {
txflag: u8, // indicates if chunked
msgtype: u8, // a flag for message type
sender: u8, // which node ID sent this frame?
routeoffset: u8, // size of array of route for frame
route: Vec<u8>, // a list of node IDs that frame should pass
payload: Vec<u8>, // payload data
}
impl Frame {
/// parse from raw bytes
pub fn from_bytes(bytes: &Vec<u8>) -> std::io::Result<Self> {
let txflag = bytes.get(0)?.clone();
let msgtype = bytes.get(1)?.clone();
let sender = bytes.get(2)?.clone();
let routesoffset = bytes.get(3)?.clone();
let routes = &bytes.get(4..(4+routesoffset as usize))?;
let (left, right) = bytes.split_at(2);
let data = Vec::from(right);
Ok(Frame {
txflag,
msgtype,
sender,
routeoffset: routesoffset,
route: Vec::from(routes),
payload: data
})
}
}
However when I try to use this pattern I get the following compilation error, and when attempting to implement the trait I get an error that the Try trait is unstable.
error[E0277]: `?` couldn't convert the error to `std::io::Error`
--> src/stack/frame.rs:121:34
|
121 | let txflag = bytes.get(0)?.clone();
| ^ the trait `std::convert::From<std::option::NoneError>` is not implemented for `std::io::Error`
Not quite sure how to proceed but I'd like to use stable features to solve this. The goal here is to be able to parse bytes and handle an invalid frame as necessary.
This is probably what you want
use std::io::{Error, ErrorKind};
fn main() {
let emptyvec = Vec::new();
match Frame::from_bytes(&emptyvec) {
Err(e) => {
println!("Received invalid frame");
}
Ok(frame) => {
println!("Received valid frame");
}
}
}
struct Frame {
txflag: u8,
// indicates if chunked
msgtype: u8,
// a flag for message type
sender: u8,
// which node ID sent this frame?
routeoffset: u8,
// size of array of route for frame
route: Vec<u8>,
// a list of node IDs that frame should pass
payload: Vec<u8>, // payload data
}
impl Frame {
/// parse from raw bytes
pub fn from_bytes(bytes: &Vec<u8>) -> std::io::Result<Self> {
let txflag = bytes.get(0).ok_or(Error::from(ErrorKind::InvalidData))?.clone();
let msgtype = bytes.get(1).ok_or(Error::from(ErrorKind::InvalidData))?.clone();
let sender = bytes.get(2).ok_or(Error::from(ErrorKind::InvalidData))?.clone();
let routesoffset = bytes.get(3).ok_or(Error::from(ErrorKind::InvalidData))?.clone();
let routes = bytes
.get(4..(4 + routesoffset as usize))
.ok_or(Error::from(ErrorKind::InvalidData))?
.clone();
let (_, right) = bytes.split_at(2);
let data = Vec::from(right);
Ok(Frame {
txflag,
msgtype,
sender,
routeoffset: routesoffset,
route: Vec::from(routes),
payload: data,
})
}
}
Here is Rust Playground
You are trying to call ? on Option. You have to convert Option to Result (If you still want to use ?).
I want to add to what Đorðe Zeljić said:
As he already pointed out the result of bytes.get(0) is a std::option::Option. When you use the ? operator on that you already left the grounds of stable Rust. This application is only supported in unstable Rust at the moment.
If you want to stay in stable Rust, it's probably best to do what Đorðe wrote. If you want to keep using the ? operator because it produces nicer looking code, here is what's going on:
Rust has a lot of error types, each being only able to represent what they are made for. If you are using a std::io::Result this implicitly uses the error type std::io::Error which is only able to represent typical I/O errors. This type is not able to represent “there was no value when I expected one”. That's why from applying ? to a Option with the None value, you don't get a std::io::Error but a different kind of error: std::option::NoneError.
When your Rust application grows it will happen often, that you have to return a Result that can contain different types of errors. In that case you normally define your own error type (enum), that can represent different kinds of errors. Then for each error, that can be contained, you have to define the From trait on your own enum. This can be a lot of repeated work, so there is a macro in the quick-error crate, that helps with that and implements the From trait automatically for each error that can be contained.
To get your code compiling, you could define the following error enum, that can represent std::io::Error as well as std::option::NoneError:
quick_error! {
#[derive(Debug)]
pub enum FrameError {
IoError(err: std::io::Error) {from() cause(err)}
MissingValue(err: std::option::NoneError) {from()}
}
}
Instead of std::io::Result<Self> your from_bytes function then has to return a std::result::Result that uses your new error type: Result<Self, FrameError>.
Completely assembled that looks like this:
#![feature(try_trait)]
use quick_error::*;
quick_error! {
#[derive(Debug)]
pub enum FrameError {
IoError(err: std::io::Error) {from() cause(err)}
MissingValue(err: std::option::NoneError) {from()}
}
}
fn main() {
let emptyvec = Vec::new();
match Frame::from_bytes(&emptyvec) {
Err(_e) => {
println!("Received invalid frame");
}
Ok(_frame) => {
println!("Received valid frame");
}
}
}
struct Frame {
txflag: u8, // indicates if chunked
msgtype: u8, // a flag for message type
sender: u8, // which node ID sent this frame?
routeoffset: u8, // size of array of route for frame
route: Vec<u8>, // a list of node IDs that frame should pass
payload: Vec<u8>, // payload data
}
impl Frame {
/// parse from raw bytes
pub fn from_bytes(bytes: &Vec<u8>) -> Result<Self, FrameError> {
let txflag = bytes.get(0)?.clone();
let msgtype = bytes.get(1)?.clone();
let sender = bytes.get(2)?.clone();
let routesoffset = bytes.get(3)?.clone();
let routes = bytes.get(4..(4 + routesoffset as usize))?;
let (left, right) = bytes.split_at(2);
let data = Vec::from(right);
Ok(Frame {
txflag,
msgtype,
sender,
routeoffset: routesoffset,
route: Vec::from(routes),
payload: data,
})
}
}
To use the quick-error crate, you have to add the following to your Cargo.toml:
[dependencies]
quick-error = "1.2.3"
I am deserializing some JSON objects which come in as requests. The input body is nested, but a certain field is sometimes misformatted for a variety of reasons. In that situation I still want the rest of the object. This doesn't all have to be done through serde; but what is happening now, is that if a single subfield is messed up, the whole request is trashed. I want to somehow still deserialize that result and just mark the field as errored out. How can this be done?
E.g. the data schema might look like:
struct BigNested {
a: Vec<A>,
b: B, // definition omitted
}
struct A {
keep_this: Foo,
trouble: SometimesBad,
}
trouble is the field that's frequently coming in messed up. I would be happy to (e.g.) turn trouble into a Result<SometimesBad, Whatever> and process it from there, but I don't know how to get serde to let me do that.
certain field is sometimes misformatted
You didn't say how malformed the incoming JSON was. Assuming it's still valid JSON, you can pull this off with Serde's struct flatten and customized deserialization:
The customized deserialization is done in a way that never fails for valid JSON input, although it may not return value of expected type if the input has unexpected format.
But these unexpected fields still need to go somewhere. Serde's struct flatten comes in handy here to catch them since any JSON snippet can be deserialized to a HashMap<String, Value>.
//# serde = { version = "1.0.103", features = ["derive"] }
//# serde_json = "1.0.44"
use serde::{Deserialize, Deserializer, de::DeserializeOwned};
use serde_json::Value;
use std::collections::HashMap;
#[derive(Deserialize, Debug)]
struct A {
keep_this: Foo,
trouble: SometimesBad,
}
#[derive(Deserialize, Debug)]
struct Foo {
foo: i32,
}
#[derive(Deserialize, Debug)]
struct SometimesBad {
inner: TryParse<Bar>,
#[serde(flatten)]
blackhole: HashMap<String, Value>,
}
#[derive(Deserialize, Debug)]
struct Bar {
bar: String,
}
#[derive(Debug)]
enum TryParse<T> {
Parsed(T),
Unparsed(Value),
NotPresent
}
impl<'de, T: DeserializeOwned> Deserialize<'de> for TryParse<T> {
fn deserialize<D: Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
match Option::<Value>::deserialize(deserializer)? {
None => Ok(TryParse::NotPresent),
Some(value) => match T::deserialize(&value) {
Ok(t) => Ok(TryParse::Parsed(t)),
Err(_) => Ok(TryParse::Unparsed(value)),
},
}
}
}
fn main() {
let valid = r#"{ "keep_this": { "foo": 1 }, "trouble": { "inner": { "bar": "one"}}}"#;
println!("{:#?}", serde_json::from_str::<A>(valid));
let extra_field = r#"{ "keep_this": { "foo": 1 }, "trouble": { "inner": { "bar": "one"}, "extra": 2019}}"#;
println!("{:#?}", serde_json::from_str::<A>(extra_field));
let wrong_type = r#"{ "keep_this": { "foo": 1 }, "trouble": { "inner": { "bar": 1}}}"#;
println!("{:#?}", serde_json::from_str::<A>(wrong_type));
let missing_field = r#"{ "keep_this": { "foo": 1 }, "trouble": { "inner": { "baz": "one"}}}"#;
println!("{:#?}", serde_json::from_str::<A>(missing_field));
let missing_inner = r#"{ "keep_this": { "foo": 1 }, "trouble": { "whatever": { "bar": "one"}}}"#;
println!("{:#?}", serde_json::from_str::<A>(missing_inner));
}
(The credit isn't all mine. Serde's issue 1583 basically has everything.)
I'm trying to implement Serialize for an enum that includes struct variants. The serde.rs documentation indicates the following:
enum E {
// Use three-step process:
// 1. serialize_struct_variant
// 2. serialize_field
// 3. end
Color { r: u8, g: u8, b: u8 },
// Use three-step process:
// 1. serialize_tuple_variant
// 2. serialize_field
// 3. end
Point2D(f64, f64),
// Use serialize_newtype_variant.
Inches(u64),
// Use serialize_unit_variant.
Instance,
}
With that in mind, I proceeded to implemention:
use serde::ser::{Serialize, SerializeStructVariant, Serializer};
use serde_derive::Deserialize;
#[derive(Deserialize)]
enum Variants {
VariantA,
VariantB { k: u32, p: f64 },
}
impl Serialize for Variants {
fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
where
S: Serializer,
{
match *self {
Variants::VariantA => serializer.serialize_unit_variant("Variants", 0, "VariantA"),
Variants::VariantB { ref k, ref p } => {
let mut state =
serializer.serialize_struct_variant("Variants", 1, "VariantB", 2)?;
state.serialize_field("k", k)?;
state.serialize_field("p", p)?;
state.end()
}
}
}
}
fn main() {
let x = Variants::VariantB { k: 5, p: 5.0 };
let toml_str = toml::to_string(&x).unwrap();
println!("{}", toml_str);
}
The code compiles, but when I run it it fails:
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: UnsupportedType', src/libcore/result.rs:999:5
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
I figured the issue must be in my use of the API, so I consulted the API documentation for StructVariant and it looks practically the same as my code. I'm sure I'm missing something, but I don't see it based on the docs and output.
Enabling external tagging for the enum enables Serde to serialize/deserialize it to TOML:
#[derive(Deserialize)]
#[serde(tag = "type")]
enum Variants {
VariantA,
VariantB { k: u32, p: f64 },
}
toml::to_string(&Variants::VariantB { k: 42, p: 13.37 })
serializes to
type = VariantB
k = 42
p = 13.37
This works well in Vecs and HashMaps, too.
The TOML format does not support enums with values:
use serde::Serialize; // 1.0.99
use toml; // 0.5.3
#[derive(Serialize)]
enum A {
B(i32),
}
fn main() {
match toml::to_string(&A::B(42)) {
Ok(s) => println!("{}", s),
Err(e) => eprintln!("Error: {}", e),
}
}
Error: unsupported Rust type
It's unclear what you'd like your data structure to map to as TOML. Using JSON works just fine:
use serde::Serialize; // 1.0.99
use serde_json; // 1.0.40
#[derive(Serialize)]
enum Variants {
VariantA,
VariantB { k: u32, p: f64 },
}
fn main() {
match serde_json::to_string(&Variants::VariantB { k: 42, p: 42.42 }) {
Ok(s) => println!("{}", s),
Err(e) => eprintln!("Error: {}", e),
}
}
{"VariantB":{"k":42,"p":42.42}}
In this minimalist program, I'd like the file_size function to include the path /not/there in the Err so it can be displayed in the main function:
use std::fs::metadata;
use std::io;
use std::path::Path;
use std::path::PathBuf;
fn file_size(path: &Path) -> io::Result<u64> {
Ok(metadata(path)?.len())
}
fn main() {
if let Err(err) = file_size(&PathBuf::from("/not/there")) {
eprintln!("{}", err);
}
}
You must define your own error type in order to wrap this additional data.
Personally, I like to use the custom_error crate for that, as it's especially convenient for dealing with several types. In your case it might look like this:
use custom_error::custom_error;
use std::fs::metadata;
use std::io;
use std::path::{Path, PathBuf};
use std::result::Result;
custom_error! {ProgramError
Io {
source: io::Error,
path: PathBuf
} = #{format!("{path}: {source}", source=source, path=path.display())},
}
fn file_size(path: &Path) -> Result<u64, ProgramError> {
metadata(path)
.map(|md| md.len())
.map_err(|e| ProgramError::Io {
source: e,
path: path.to_path_buf(),
})
}
fn main() {
if let Err(err) = file_size(&PathBuf::from("/not/there")) {
eprintln!("{}", err);
}
}
Output:
/not/there: No such file or directory (os error 2)
While Denys Séguret's answer is correct, I like using my crate SNAFU because it provides the concept of a context. This makes the act of attaching the path (or anything else!) very easy to do:
use snafu::{ResultExt, Snafu}; // 0.2.3
use std::{
fs, io,
path::{Path, PathBuf},
};
#[derive(Debug, Snafu)]
enum ProgramError {
#[snafu(display("Could not get metadata for {}: {}", path.display(), source))]
Metadata { source: io::Error, path: PathBuf },
}
fn file_size(path: impl AsRef<Path>) -> Result<u64, ProgramError> {
let path = path.as_ref();
let md = fs::metadata(&path).context(Metadata { path })?;
Ok(md.len())
}
fn main() {
if let Err(err) = file_size("/not/there") {
eprintln!("{}", err);
}
}