I'm looking for a way to initialize a structopt Vec field with multiple items by default. I can do it for a single item with:
use structopt::StructOpt;
#[derive(Debug, StructOpt)]
pub struct Cli {
#[structopt(default_value = "foo")]
foo: Vec<String>,
}
fn main() {
let cli = Cli::from_iter(Vec::<String>::new());
assert_eq!(cli.foo, vec!["foo"]);
}
But how to make cli.foo to be equal let's say vec!["foo", "bar"] by default?
I've followed the L. Riemer advice, and it seems it's enough to implement FromStr only:
use structopt::StructOpt;
#[derive(Debug, PartialEq)]
struct Foo(Vec<String>);
impl std::str::FromStr for Foo {
type Err = Box<dyn std::error::Error>;
fn from_str(s: &str) -> Result<Self, Self::Err> {
Ok(Foo(s.split(",").map(|x| x.trim().to_owned()).collect()))
}
}
#[derive(StructOpt)]
pub struct Cli {
#[structopt(long, default_value = "foo, bar")]
foo: Foo,
}
fn main() {
let cli = Cli::from_iter(Vec::<String>::new());
assert_eq!(cli.foo, Foo(vec!["foo".into(), "bar".into()]));
let cli = Cli::from_iter(vec!["", "--foo", "foo"]);
assert_eq!(cli.foo, Foo(vec!["foo".into()]));
let cli = Cli::from_iter(vec!["", "--foo", "foo,bar,baz"]);
assert_eq!(cli.foo, Foo(vec!["foo".into(), "bar".into(), "baz".into()]));
}
I don't think you can do that: while StructOpt has some tricks around default values, I expect this still ends with the default value being injected in the CLI parsing as if it had been provided explicitly, which means there is likely no way to provide multiple default values (though I could certainly be wrong).
You probably want to handle this at the application-level e.g. right after having parsed the CLI, check for the arity of foo and update it if it's empty.
Related
I'm confused why I am getting an Err(missing field "web3_node_provider") error when I cargo run using config-rs. Appears to fail at s.try_deserialize():
use config::{Config, ConfigError, Environment, File};
use serde::Deserialize;
#[derive(Debug, Deserialize)]
#[allow(unused)]
struct Web3NodeProvider {
ethereum_mainnet_node_url_http: String,
alchemy_api_key: String,
}
#[derive(Debug, Deserialize)]
#[allow(unused)]
pub struct Settings {
web3_node_provider: Web3NodeProvider,
}
impl Settings {
pub fn new() -> Result<Self, ConfigError> {
let s = Config::builder()
.add_source(File::with_name("config/default"))
.add_source(File::with_name("config/local").required(false))
.add_source(Environment::with_prefix("app"))
.build()?;
s.try_deserialize()
}
}
fn main() {
let settings = Settings::new();
println!("{:?}", settings);
}
I've pretty much followed the hierarchy example in config-rs, so I'm sure I'm just misunderstanding something basic or missing something. I am able to use "Web3NodeProvider.url" but not "web3_node_provider.ethereum_mainnet_node_url_http".
default.toml
[Web3NodeProvider]
ethereum_mainnet_node_url_http = "https://eth-mainnet.g.alchemy.com/v2/"
alchemy_api_key = "alchemy-api-key"
local.toml
[Web3NodeProvider]
alchemy_api_key = "randomapikey"
As per example, you have to name your field in config as name of attribute not name of structure (type). Like so:
[web3_node_provider]
ethereum_mainnet_node_url_http = "https://eth-mainnet.g.alchemy.com/v2/"
alchemy_api_key = "alchemy-api-key"
I'm trying to access an api, where I can specify what kind of fields I want included in the result. (for example "basic", "advanced", "irrelevant"
the Rust Struct to represent that would look something like
Values {
a: Option<String>;
b: Option<String>;
c: Option<String>;
d: Option<String>;
}
or probably better:
Values {
a: Option<Basic>; // With field a
b: Option<Advanced>; // With fields b,c
c: Option<Irrelevant>; // With field d
}
Using this is possible, but I'd love to reduce the handling of Option for the caller.
Is it possible to leverage the type system to simplify the usage? (Or any other way I'm not realizing?)
My idea was something in this direction, but I think that might not be possible with rust (at least without macros):
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=093bdf1853978af61443d547082576ca
struct Values {
a: Option<&'static str>,
b: Option<&'static str>,
c: Option<&'static str>,
}
trait ValueTraits{}
impl ValueTraits for dyn Basic{}
impl ValueTraits for dyn Advanced{}
impl ValueTraits for Values{}
trait Basic {
fn a(&self) -> &'static str;
}
trait Advanced {
fn b(&self) -> &'static str;
fn c(&self) -> &'static str;
}
impl Basic for Values {
fn a(&self) -> &'static str {
self.a.unwrap()
}
}
impl Advanced for Values {
fn b(&self) -> &'static str {
self.b.unwrap()
}
fn c(&self) -> &'static str {
self.c.unwrap()
}
}
//Something like this is probably not possible, as far as I understand Rust
fn get_values<T1, T2>() -> T1 + T2{
Values {
a: "A",
b: "B",
c: "C"
}
}
fn main() {
let values = get_values::<Basic, Advanced>();
println!("{}, {}, {}", values.a(), values.b(), values.c());
}
Clarifications (Edit)
The Values struct contains deserialized json data from the api I called. I can request groups of fields to be included in the response(1-n requested fields groups), the fields are of different types.
If I knew beforehand, which of those fields are returned, I wouldn't need them to be Option, but as the caller decides which fields are returned, the fields needs to be Option (either directly, or grouped by the field groups)
There are too many possible combinations to create a struct for each of those.
I fully realize that this cannot work, it was just "peudorust":
fn get_values<T1, T2>() -> T1 + T2{
Values {
a: "A",
b: "B",
c: "C"
}
}
But my thought process was:
In theory, I could request the field groups via generics, so I could create a "dynamic" type, that implements these traits, because I know which traits are requested.
The Traits are supposed to act like a "view" into the actual struct, because if they are requested beforehand, I know I should request them from the api to include them in the Struct.
My knowledge of generics and traits isn't enough to confidently say "this isn't possible at all" and I couldn't find a conclusive answer before I asked here.
Sorry for the initial question not being clear of what the actual issue was, I hope the clarification helps with that.
I can't quite gauge whether or not you want to be able to request and return fields of multiple different types from the question. But if all the information being returned is of a single type you could try using a HashMap:
use std::collections::HashMap;
fn get_values(fields: &[&'static str]) -> HashMap<&'static str, &'static str> {
let mut selected = HashMap::new();
for field in fields {
let val = match *field {
"a" => "Value of a",
"b" => "Value of b",
"c" => "Value of c",
// Skip requested fields that don't exist.
_ => continue,
};
selected.insert(*field, val);
}
selected
}
fn main() {
let fields = ["a","c"];
let values = get_values(&fields);
for (field, value) in values.iter() {
println!("`{}` = `{}`", field, value);
}
}
Additionally you've given me the impression that you haven't quite been able to form a relationship between generics and traits yet. I highly recommend reading over the book's "Generic Types, Traits, and Lifetimes" section.
The gist of it is that generics exist to generalize a function, struct, enum, or even a trait to any type, and traits are used to assign behaviour to a type. Traits cannot be passed as a generic parameter, because traits are not types, they are behaviours. Which is why doing: get_values::<Basic, Advanced>(); doesn't work. Basic and Advanced are both traits, not types.
If you want practice with generics try generalizing get_values so that it can accept any type which can be converted into an iterator that yields &'static strs.
Edit:
The clarification is appreciated. The approach you have in mind is possible, but I wouldn't recommend it because it's implementing it is extremely verbose and will panic the moment the format of the json you're parsing changes. Though if you really need to use traits for some reason you could try something like this:
// One possible construct returned to you.
struct All {
a: Option<i32>,
b: Option<i32>,
c: Option<i32>,
}
// A variation that only returned b and c
struct Bc {
b: Option<i32>,
c: Option<i32>,
}
// impl Advanced + Basic + Default for All {...}
// impl Advanced + Default for Bc {...}
fn get_bc<T: Advanced + Default>() -> T {
// Here you would set the fields on T.
Default::default()
}
fn get_all<T: Basic + Advanced + Default>() -> T {
Default::default()
}
fn main() {
// This isn't really useful unless you want to create multiple structs that
// are supposed to hold b and c but otherwise have different fields.
let bc = get_bc::<Bc>();
let all = get_all::<All>();
// Could also do something like:
let bc = get_bc::<All>();
// but that could get confusing.
}
I think the above is how you're looking to solve your problem. Though if you can, I would still recommend using a HashMap with a trait object like this:
use std::collections::HashMap;
use std::fmt::Debug;
// Here define the behaviour you need to interact with the data. In this case
// it's just ability to print to console.
trait Value: Debug {}
impl Value for &'static str {}
impl Value for i32 {}
impl<T: Debug> Value for Vec<T> {}
fn get_values(fields: &[&'static str]) -> HashMap<&'static str, Box<dyn Value>> {
let mut selected = HashMap::new();
for field in fields {
let val = match *field {
"a" => Box::new("Value of a") as Box<dyn Value>,
"b" => Box::new(2) as Box<dyn Value>,
"c" => Box::new(vec![1,3,5,7]) as Box<dyn Value>,
// Skip requested fields that don't exist.
_ => continue,
};
selected.insert(*field, val);
}
selected
}
fn main() {
let fields = ["a","c"];
let values = get_values(&fields);
for (field, value) in values.iter() {
println!("`{}` = `{:?}`", field, value);
}
}
Wondering if there's a "proper" way of converting an Enum to a &str and back.
The problem I'm trying to solve:
In the clap crate, args/subcommands are defined and identified by &strs. (Which I'm assuming don't fully take advantage of the type checker.) I'd like to pass a Command Enum to my application instead of a &str which would be verified by the type-checker and also save me from typing (typo-ing?) strings all over the place.
This is what I came up with from searching StackOverflow and std:
use std::str::FromStr;
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum Command {
EatCake,
MakeCake,
}
impl FromStr for Command {
type Err = ();
fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
match s.to_ascii_lowercase().as_str() {
"eat-cake" => Ok(Self::EatCake),
"make-cake" => Ok(Self::MakeCake),
_ => Err(()),
}
}
}
impl<'a> From<Command> for &'a str {
fn from(c: Command) -> Self {
match c {
Command::EatCake => "eat-cake",
Command::MakeCake => "make-cake",
}
}
}
fn main() {
let command_from_str: Command = "eat-cake".to_owned().parse().unwrap();
let str_from_command: &str = command_from_str.into();
assert_eq!(command_from_str, Command::EatCake);
assert_eq!(str_from_command, "eat-cake");
}
And here's a working playground:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b5e9ac450fd6a79b855306e96d4707fa
Here's an abridged version of what I'm running in clap.
let matches = App::new("cake")
.setting(AppSettings::SubcommandRequiredElseHelp)
// ...
.subcommand(
SubCommand::with_name(Command::MakeCake.into())
// ...
)
.subcommand(
SubCommand::with_name(Command::EatCake.into())
// ...
)
.get_matches();
It seems to work, but I'm not sure if I'm missing something / a bigger picture.
Related:
How to use an internal library Enum for Clap Args
How do I return an error within match statement while implementing from_str in rust?
Thanks!
The strum crate may save you some work. Using strum I was able to get the simple main() you have to work without any additional From implementations.
use strum_macros::{Display, EnumString, IntoStaticStr};
#[derive(Debug, Clone, Copy, PartialEq)]
#[derive(Display, EnumString, IntoStaticStr)] // strum macros.
pub enum Command {
#[strum(serialize = "eat-cake")]
EatCake,
#[strum(serialize = "make-cake")]
MakeCake,
}
fn main() {
let command_from_str: Command = "eat-cake".to_owned().parse().unwrap();
let str_from_command: &str = command_from_str.into();
assert_eq!(command_from_str, Command::EatCake);
assert_eq!(str_from_command, "eat-cake");
}
I want to convert multiple env.variables to static struct.
I can do it mannually:
Env {
is_development: env::var("IS_DEVELOPMENT")
.unwrap()
.parse::<bool>()
.unwrap(),
server: Server {
host: env::var("HOST").unwrap(),
port: env::var("PORT")
.unwrap()
.parse::<u16>()
.unwrap(),
},
}
But when there is multiple values, it's became bloated. Is there a way to make generic helper function that will give me value that i specify or panic? Something like this (or another solution):
fn get_env_var<T>(env_var_name: String) -> T {
// panic is ok here
let var = env::var(env_var_name).unwrap();
T::from(var)
}
get_env_var<u16>("PORT") // here i got u16
get_env_var<bool>("IS_DEVELOPMENT") // here is my boolean
Full example
use crate::server::logger::log_raw;
use dotenv::dotenv;
use serde::Deserialize;
use std::env;
#[derive(Deserialize, Debug, Clone)]
pub struct Server {
pub host: String,
pub port: u16,
}
#[derive(Deserialize, Debug, Clone)]
pub struct Env {
pub is_development: bool,
pub server: Server,
}
impl Env {
pub fn init() -> Self {
dotenv().expect(".env loading fail");
// how can i specify what type i expect?
fn get_env_var<T>(env_var_name: String) -> T {
// panic is ok here
let var = env::var(env_var_name).unwrap();
T::from(var)
}
// instead this
Env {
is_development: env::var("IS_DEVELOPMENT")
.unwrap()
.parse::<bool>()
.unwrap(),
server: Server {
host: env::var("HOST").unwrap(),
port: env::var("PORT")
.unwrap()
.parse::<u16>()
.unwrap(),
},
}
// do something like this
/*
Env {
is_development: get_env_var<bool>("IS_DEVELOPMENT"),
server: Server {
host: get_env_var<String>("HOST"),
port: get_env_var<u16>("PORT"),
},
}
*/
}
}
lazy_static! {
pub static ref ENV: Env = Env::init();
}
Like in your manual version, where you use str::parse, you can have the same requirement as str::parse, which is FromStr. So if you include the T: FromStr requirement, then you'll be able to do var.parse::<T>().
use std::env;
use std::fmt::Debug;
use std::str::FromStr;
fn get_env_var<T>(env_var_name: &str) -> T
where
T: FromStr,
T::Err: Debug,
{
let var = env::var(env_var_name).unwrap();
var.parse::<T>().unwrap()
}
Then if you run the following by executing PORT=1234 IS_DEVELOPMENT=true cargo run.
fn main() {
println!("{}", get_env_var::<u16>("PORT"));
println!("{}", get_env_var::<bool>("IS_DEVELOPMENT"));
}
Then it will output:
1234
true
Alternatively, you might want to be able to handle VarError::NotPresent and fallback to a default.
use std::env::{self, VarError};
use std::fmt::Debug;
use std::str::FromStr;
fn get_env_var<T>(env_var_name: &str) -> Result<T, VarError>
where
T: FromStr,
T::Err: Debug,
{
let var = env::var(env_var_name)?;
Ok(var.parse().unwrap())
}
Now if you only executed PORT=1234 cargo run, then it would make it easier to do this:
let is_dev = get_env_var::<bool>("IS_DEVELOPMENT")
.map_err(|err| match err {
VarError::NotPresent => Ok(false),
err => Err(err),
})
.unwrap();
println!("{:?}", is_dev);
If you want to fallback to Default if VarError::NotPresent:
fn get_env_var<T>(env_var_name: &str) -> T
where
T: FromStr,
T::Err: Debug,
T: Default,
{
let var = match env::var(env_var_name) {
Err(VarError::NotPresent) => return T::default(),
res => res.unwrap(),
};
var.parse().unwrap()
}
Rust genericity, inspired by Haskell's works through traits and specifically trait bounds. This means when you write
fn get_env_var<T>(env_var_name: String) -> T
since there is no trait bound on T there are essentially no capabilities for it (this is rather unlike C++).
Therefore, as far as rustc is concerned, pretty much the only thing it can do with a T is... take one as parameter then return it as-is.
Thus to do anything useful with a T (including creating one, whether from something else or de novo) you need to use the correct trait and provide the correct trait bounds.
The From trait is entirely the wrong trait to involve here: it specifies total (never-failing) conversions e.g. converting a u16 to a u32, which can never fail.
Whether it's converting a String to a bool or a u16, the conversion is quite obviously less than total: there is an infinity of string values which are not sequences of decimal digits describing a number below 2^16.
In Rust, the signifier of failabibility tends to be Try. There is a TryFrom trait, however for historical reasons and as it documents in its signature the str::parse method is hooked on the FromStr trait.
This means in order to declare that your T can be created from a string (and use the parse method to create one), you need to bound T to FromStr. And of course indicate that it may fail, and will return whatever error T generates when it can't be parsed from a string:
fn get_env_var<T: FromStr>(env_var_name: String) -> Result<T, T::Err> {
let var = env::var(env_var_name).unwrap();
var.parse()
}
Incidentally, taking a String as input is usually avoided unless you really have to[0]. Usually you'd take an &str, that's a lot more flexible as it can be used e.g. with string literals (which are of type &'static str).
So
fn get_env_var<T: FromStr>(env_var_name: &str) -> Result<T, T::Err> {
let var = env::var(env_var_name).unwrap();
var.parse()
}
[0] or for efficiency purposes sometimes
I've met a conflict with Rust's ownership rules and a trait object downcast. This is a sample:
use std::any::Any;
trait Node{
fn gen(&self) -> Box<Node>;
}
struct TextNode;
impl Node for TextNode{
fn gen(&self) -> Box<Node>{
Box::new(TextNode)
}
}
fn main(){
let mut v: Vec<TextNode> = Vec::new();
let node = TextNode.gen();
let foo = &node as &Any;
match foo.downcast_ref::<TextNode>(){
Some(n) => {
v.push(*n);
},
None => ()
};
}
The TextNode::gen method has to return Box<Node> instead of Box<TextNode>, so I have to downcast it to Box<TextNode>.
Any::downcast_ref's return value is Option<&T>, so I can't take ownership of the downcast result and push it to v.
====edit=====
As I am not good at English, my question is vague.
I am implementing (copying may be more precise) the template parser in Go standard library.
What I really need is a vector, Vec<Box<Node>> or Vec<Box<Any>>, which can contain TextNode, NumberNode, ActionNode, any type of node that implements the trait Node can be pushed into it.
Every node type needs to implement the copy method, return Box<Any>, and then downcasting to the concrete type is OK. But to copy Vec<Box<Any>>, as you don't know the concrete type of every element, you have to check one by one, that is really inefficient.
If the copy method returns Box<Node>, then copying Vec<Box<Node>> is simple. But it seems that there is no way to get the concrete type from trait object.
If you control trait Node you can have it return a Box<Any> and use the Box::downcast method
It would look like this:
use std::any::Any;
trait Node {
fn gen(&self) -> Box<Any>; // downcast works on Box<Any>
}
struct TextNode;
impl Node for TextNode {
fn gen(&self) -> Box<Any> {
Box::new(TextNode)
}
}
fn main() {
let mut v: Vec<TextNode> = Vec::new();
let node = TextNode.gen();
if let Ok(n) = node.downcast::<TextNode>() {
v.push(*n);
}
}
Generally speaking, you should not jump to using Any. I know it looks familiar when coming from a language with subtype polymorphism and want to recreate a hierarchy of types with some root type (like in this case: you're trying to recreate the TextNode is a Node relationship and create a Vec of Nodes). I did it too and so did many others: I bet the number of SO questions on Any outnumbers the times Any is actually used on crates.io.
While Any does have its uses, in Rust it has alternatives.
In case you have not looked at them, I wanted to make sure you considered doing this with:
enums
Given different Node types you can express the "a Node is any of these types" relationship with an enum:
struct TextNode;
struct XmlNode;
struct HtmlNode;
enum Node {
Text(TextNode),
Xml(XmlNode),
Html(HtmlNode),
}
With that you can put them all in one Vec and do different things depending on the variant, without downcasting:
let v: Vec<Node> = vec![
Node::Text(TextNode),
Node::Xml(XmlNode),
Node::Html(HtmlNode)];
for n in &v {
match n {
&Node::Text(_) => println!("TextNode"),
&Node::Xml(_) => println!("XmlNode"),
&Node::Html(_) => println!("HtmlNode"),
}
}
playground
adding a variant means potentially changing your code in many places: the enum itself and all the functions that do something with the enum (to add the logic for the new variant). But then again, with Any it's mostly the same, all those functions might need to add the downcast to the new variant.
Trait objects (not Any)
You can try putting the actions you'd want to perform on the various types of nodes in the trait, so you don't need to downcast, but just call methods on the trait object.
This is essentially what you were doing, except putting the method on the Node trait instead of downcasting.
playground
The (more) ideomatic way for the problem:
use std::any::Any;
pub trait Nodeable {
fn as_any(&self) -> &dyn Any;
}
#[derive(Clone, Debug)]
struct TextNode {}
impl Nodeable for TextNode {
fn as_any(&self) -> &dyn Any {
self
}
}
fn main() {
let mut v: Vec<Box<dyn Nodeable>> = Vec::new();
let node = TextNode {}; // or impl TextNode::new
v.push(Box::new(node));
// the downcast back to TextNode could be solved like this:
if let Some(b) = v.pop() { // only if we have a node…
let n = (*b).as_any().downcast_ref::<TextNode>().unwrap(); // this is secure *)
println!("{:?}", n);
};
}
*) This is secure: only Nodeables are allowd to be downcasted to types that had Nodeable implemented.