Can you put another match clause in one of the match results of a match like this in:
pub fn is_it_file(input_file: &str) -> String {
let path3 = Path::new(input_file);
match path3.is_file() {
true => "File!".to_string(),
false => match path3.is_dir() {
true => "Dir!".to_string(),
_ => "Don't care",
}
}
}
If not why ?
Yes you can (see Qantas' answer). But Rust often has prettier ways to do what you want. You can do multiple matches at once by using tuples.
pub fn is_it_file(input_file: &str) -> String {
let path3 = Path::new(input_file);
match (path3.is_file(), path3.is_dir()) {
(true, false) => "File!",
(false, true) => "Dir!",
_ => "Neither or Both... bug?",
}.to_string()
}
Sure you can, match is an expression:
fn main() {
fn foo() -> i8 {
let a = true;
let b = false;
match a {
true => match b {
true => 1,
false => 2
},
false => 3
}
}
println!("{}", foo()); // 2
}
You can view the results of this on the Rust playpen.
The only thing that seems off about your code to me is the inconsistent usage of .to_string() in your code, the last match case doesn't have that.
Related
How can I convert this loop based implementation to iteration syntax?
fn parse_number<B: AsRef<str>>(input: B) -> Option<u32> {
let mut started = false;
let mut b = String::with_capacity(50);
let radix = 16;
for c in input.as_ref().chars() {
match (started, c.is_digit(radix)) {
(false, false) => {},
(false, true) => {
started = true;
b.push(c);
},
(true, false) => {
break;
}
(true, true) => {
b.push(c);
},
}
}
if b.len() == 0 {
None
} else {
match u32::from_str_radix(b.as_str(), radix) {
Ok(v) => Some(v),
Err(_) => None,
}
}
}
The main problem that I found is that you need to terminate the iterator early and be able to ignore characters until the first numeric char is found.
.map_while() fails because it has no state.
.reduce() and .fold() would iterate over the entire str regardless if the number has already ended.
It looks like you want to find the first sequence of digits while ignoring any non-digits before that. You can use a combination of .skip_while and .take_while:
fn parse_number<B: AsRef<str>>(input: B) -> Option<u32> {
let input = input.as_ref();
let radix = 10;
let digits: String = input.chars()
.skip_while(|c| !c.is_digit(radix))
.take_while(|c| c.is_digit(radix))
.collect();
u32::from_str_radix(&digits, radix).ok()
}
fn main() {
dbg!(parse_number("I have 52 apples"));
}
[src/main.rs:14] parse_number("I have 52 apples") = Some(
52,
)
How do I check if a vector of enum has a certain member with values?
#[derive(PartialEq,Clone,Copy)]
enum TestResult {
Pass,
Fail {point: u8},
}
impl TestResult {
fn is_fail(&self) -> bool {
match *self {
TestResult::Fail{point:_} => true,
_ => false,
}
}
}
fn main() {
let v1 = vec![TestResult::Pass,
TestResult::Pass,
TestResult::Fail{point:50}];
if v1.contains(&TestResult::Pass) {
println!("result contains Pass");
}
if v1.iter().any(|&r| r.is_fail()) {
println!("result contains Fail");
}
}
This is working but is there a way to do this with Vec::contains()?
I want to check if TestResult::Fail is in a vector in the same way as for TestResult::Pass (no pattern matching. easy..)
There isn't really an easy way to do this, however we can make a simple macro to perform some pattern matching in the if statement.
/// Performs pattern matching contains on an IntoIter type
macro_rules! matches_any {
($iterable:expr, $($tokens:tt)+) => {{
let iter = ::std::iter::IntoIterator::into_iter($iterable);
iter.any(|x| matches!(x, $($tokens)+))
}};
}
The trick here is that we can use the matches! macro instead of a full match statement if all we want to know is if something matches a given pattern.
// Without matches!
match self {
TestResult::Fail {..} => true,
_ => false,
}
// With matches!
matches!(self, TestResult::Fail {..})
So now we can use this macro instead:
let v1 = vec![TestResult::Pass,
TestResult::Pass,
TestResult::Fail { point: 50 }];
if matches_any!(&v1, TestResult::Pass) {
println!("result contains Pass");
}
if matches_any!(&v1, TestResult::Fail {..}) {
println!("result contains Fail");
}
I am confused about the Some(T) keyword.
I want to check for two variables, if the value is defined (not None). If that is the case, the value of this variables is processed.
I know the match pattern which works like this:
match value {
Some(val) => println!("{}", val),
None => return false,
}
If I use this pattern, it will get very messy:
match param {
Some(par) => {
match value {
Some(val) => {
//process
},
None => return false,
}
},
None => return false,
}
This can't be the right solution.
The is a possibility, to ask if the param and value is_some() That would effect code like that:
if param.is_some() && value.is_some() {
//process
}
But if I do it like that, I always have to unwrap param and value to access the values.
I thought about something like this to avoid that. But this code does not work:
if param == Some(par) && value == Some(val) {
//process
}
The idea is that the values are accessible by par and val like they are in the match version.
Is there any solution to do something like this?
If I have several Option values to match, I match on a tuple of the values:
enum Color {
Red,
Blue,
Green,
}
fn foo(a: Option<Color>, b: Option<i32>) {
match (a, b) {
(Some(Color::Blue), Some(n)) if n > 10 => println!("Blue large number"),
(Some(Color::Red), _) => println!("Red number"),
_ => (),
}
}
fn main() {
foo(Some(Color::Blue), None);
foo(Some(Color::Blue), Some(20));
}
This allows me to match the combinations that are interesting, and discard the rest (or return false, if that is what you want to do).
If your function is processing multiple Option values, and would like to discard them if they're not Some, your function could return an Option itself:
fn foo(param: Option<usize>, value: Option<usize>) -> Option<usize> {
let result = param? + value?;
Some(result)
}
This will short-circuit the function in case there's a None value stored in either param or value.
Please read the book for more information on the ? operator.
If your function can't return an Option, you can still get away with destructuring using if let or match:
let x = if let (Some(p), Some(v)) = (param, value) {
p + v
} else {
return 0;
}
let x = match (param, value) {
(Some(p), Some(v)) => p + v,
(Some(p), _) => p,
(_, Some(v) => v,
_ => return 0,
}
Please read What is this question mark operator about? for more information on the ? operator
Please read this chapter in Rust by Example for more information on destructuring multiple things at once
There's a couple more alternatives not yet listed:
If you're willing to use experimental features (and hence the nightly compiler) you can use a try block as an alternative of extracting a function.
#![feature(try_blocks)]
fn main() {
let par: Option<f32> = Some(1.0f32);
let value: Option<f32> = Some(2.0f32);
let x: Option<f32> = try { par? + value? };
println!("{:?}", x);
}
Another alternative is to use map which only applies if the value is not None
let x: Option<f32> = par.map(|p| value.map(|v| p + v));
I'm using syn to parse Rust code. When I read a named field's type using field.ty, I get a syn::Type. When I print it using quote!{#ty}.to_string() I get "Option<String>".
How can I get just "String"? I want to use #ty in quote! to print "String" instead of "Option<String>".
I want to generate code like:
impl Foo {
pub set_bar(&mut self, v: String) {
self.bar = Some(v);
}
}
starting from
struct Foo {
bar: Option<String>
}
My attempt:
let ast: DeriveInput = parse_macro_input!(input as DeriveInput);
let data: Data = ast.data;
match data {
Data::Struct(ref data) => match data.fields {
Fields::Named(ref fields) => {
fields.named.iter().for_each(|field| {
let name = &field.ident.clone().unwrap();
let ty = &field.ty;
quote!{
impl Foo {
pub set_bar(&mut self, v: #ty) {
self.bar = Some(v);
}
}
};
});
}
_ => {}
},
_ => panic!("You can derive it only from struct"),
}
My updated version of the response from #Boiethios, tested and used in a public crate, with support of several syntaxes for Option:
Option
std::option::Option
::std::option::Option
core::option::Option
::core::option::Option
fn extract_type_from_option(ty: &syn::Type) -> Option<&syn::Type> {
use syn::{GenericArgument, Path, PathArguments, PathSegment};
fn extract_type_path(ty: &syn::Type) -> Option<&Path> {
match *ty {
syn::Type::Path(ref typepath) if typepath.qself.is_none() => Some(&typepath.path),
_ => None,
}
}
// TODO store (with lazy static) the vec of string
// TODO maybe optimization, reverse the order of segments
fn extract_option_segment(path: &Path) -> Option<&PathSegment> {
let idents_of_path = path
.segments
.iter()
.into_iter()
.fold(String::new(), |mut acc, v| {
acc.push_str(&v.ident.to_string());
acc.push('|');
acc
});
vec!["Option|", "std|option|Option|", "core|option|Option|"]
.into_iter()
.find(|s| &idents_of_path == *s)
.and_then(|_| path.segments.last())
}
extract_type_path(ty)
.and_then(|path| extract_option_segment(path))
.and_then(|path_seg| {
let type_params = &path_seg.arguments;
// It should have only on angle-bracketed param ("<String>"):
match *type_params {
PathArguments::AngleBracketed(ref params) => params.args.first(),
_ => None,
}
})
.and_then(|generic_arg| match *generic_arg {
GenericArgument::Type(ref ty) => Some(ty),
_ => None,
})
}
You should do something like this untested example:
use syn::{GenericArgument, PathArguments, Type};
fn extract_type_from_option(ty: &Type) -> Type {
fn path_is_option(path: &Path) -> bool {
leading_colon.is_none()
&& path.segments.len() == 1
&& path.segments.iter().next().unwrap().ident == "Option"
}
match ty {
Type::Path(typepath) if typepath.qself.is_none() && path_is_option(typepath.path) => {
// Get the first segment of the path (there is only one, in fact: "Option"):
let type_params = typepath.path.segments.iter().first().unwrap().arguments;
// It should have only on angle-bracketed param ("<String>"):
let generic_arg = match type_params {
PathArguments::AngleBracketed(params) => params.args.iter().first().unwrap(),
_ => panic!("TODO: error handling"),
};
// This argument must be a type:
match generic_arg {
GenericArgument::Type(ty) => ty,
_ => panic!("TODO: error handling"),
}
}
_ => panic!("TODO: error handling"),
}
}
There's not many things to explain, it just "unrolls" the diverse components of a type:
Type -> TypePath -> Path -> PathSegment -> PathArguments -> AngleBracketedGenericArguments -> GenericArgument -> Type.
If there is an easier way to do that, I would be happy to know it.
Note that since syn is a parser, it works with tokens. You cannot know for sure that this is an Option. The user could, for example, type std::option::Option, or write type MaybeString = std::option::Option<String>;. You cannot handle those arbitrary names.
use std::cmp::Ordering;
fn cmp(a: i32, b: i32) -> Ordering {
match {
_ if a < b => Ordering::Less,
_ if a > b => Ordering::Greater,
_ => Ordering::Equal,
}
}
fn main() {
let x = 5;
let y = 10;
println!("{}", match cmp(x, y) {
Ordering::Less => "less",
Ordering::Greater => "greater",
Ordering::Equal => "equal",
});
}
How to use match with conditions, without destructuring (because there's nothing to destructure), in the function cmp above?
The code has been adapted from the well-known example in the book which uses only if/else, however, it does not work:
src/main.rs:5:9: 5:10 error: unexpected token: `_`
src/main.rs:5 _ if a < b => Ordering::Less,
^
Could not compile `match_ordering`.
I am using rustc 1.0.0-nightly (3ef8ff1f8 2015-02-12 00:38:24 +0000).
This would work:
fn cmp(a: i32, b: i32) -> Ordering {
match (a, b) {
(a,b) if a < b => Ordering::Less,
(a,b) if a > b => Ordering::Greater,
_ => Ordering::Equal,
}
}
but it would use destructuring. Is there any other way, or is this just the most idiomatic, clean way of writing it?
You need to match on something, i.e. match { ... } isn't valid because there needs to be something between the match and the {. Since you don't care about the value itself, matching on unit, (), should be fine: match () { ... }, e.g.:
match () {
_ if a < b => Ordering::Less,
_ if a > b => Ordering::Greater,
_ => Ordering::Equal
}
(To be strict: match { _ => ... } is actually trying to parse the { ... } as the match head (i.e. the expression being matched on), and _ isn't valid at the start of an expression, hence the error.)
On idiomacity: I personally think expressing a series of conditions with short results (like this) is fine with match and a series of _ if ...s as you have done.