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Merge pull request #772 from tlyu/errors-rework

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feature: improve error_handling exercises
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marisa 2021-06-24 14:12:34 +02:00 committed by GitHub
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5 changed files with 196 additions and 155 deletions
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4
exercises/error_handling/result1.rs → exercises/error_handling/errors4.rs
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@ -1,5 +1,5 @@
// result1.rs
// Make this test pass! Execute `rustlings hint result1` for hints :)
// errors4.rs
// Make this test pass! Execute `rustlings hint errors4` for hints :)
// I AM NOT DONE

53
exercises/error_handling/errors5.rs Normal file
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@ -0,0 +1,53 @@
// errors5.rs
// This program uses a completed version of the code from errors4.
// It won't compile right now! Why?
// Execute `rustlings hint errors5` for hints!
// I AM NOT DONE
use std::error;
use std::fmt;
use std::num::ParseIntError;
// TODO: update the return type of `main()` to make this compile.
fn main() -> Result<(), ParseIntError> {
let pretend_user_input = "42";
let x: i64 = pretend_user_input.parse()?;
println!("output={:?}", PositiveNonzeroInteger::new(x)?);
Ok(())
}
// Don't change anything below this line.
#[derive(PartialEq, Debug)]
struct PositiveNonzeroInteger(u64);
#[derive(PartialEq, Debug)]
enum CreationError {
Negative,
Zero,
}
impl PositiveNonzeroInteger {
fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
match value {
x if x < 0 => Err(CreationError::Negative),
x if x == 0 => Err(CreationError::Zero),
x => Ok(PositiveNonzeroInteger(x as u64))
}
}
}
// This is required so that `CreationError` can implement `error::Error`.
impl fmt::Display for CreationError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let description = match *self {
CreationError::Negative => "number is negative",
CreationError::Zero => "number is zero",
};
f.write_str(description)
}
}
impl error::Error for CreationError {}

95
exercises/error_handling/errors6.rs Normal file
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@ -0,0 +1,95 @@
// errors6.rs
// Using catch-all error types like `Box<dyn error::Error>` isn't recommended
// for library code, where callers might want to make decisions based on the
// error content, instead of printing it out or propagating it further. Here,
// we define a custom error type to make it possible for callers to decide
// what to do next when our function returns an error.
// Make these tests pass! Execute `rustlings hint errors6` for hints :)
// I AM NOT DONE
use std::num::ParseIntError;
// This is a custom error type that we will be using in `parse_pos_nonzero()`.
#[derive(PartialEq, Debug)]
enum ParsePosNonzeroError {
Creation(CreationError),
ParseInt(ParseIntError)
}
impl ParsePosNonzeroError {
fn from_creation(err: CreationError) -> ParsePosNonzeroError {
ParsePosNonzeroError::Creation(err)
}
// TODO: add another error conversion function here.
}
fn parse_pos_nonzero(s: &str)
-> Result<PositiveNonzeroInteger, ParsePosNonzeroError>
{
// TODO: change this to return an appropriate error instead of panicking
// when `parse()` returns an error.
let x: i64 = s.parse().unwrap();
PositiveNonzeroInteger::new(x)
.map_err(ParsePosNonzeroError::from_creation)
}
// Don't change anything below this line.
#[derive(PartialEq, Debug)]
struct PositiveNonzeroInteger(u64);
#[derive(PartialEq, Debug)]
enum CreationError {
Negative,
Zero,
}
impl PositiveNonzeroInteger {
fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
match value {
x if x < 0 => Err(CreationError::Negative),
x if x == 0 => Err(CreationError::Zero),
x => Ok(PositiveNonzeroInteger(x as u64))
}
}
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn test_parse_error() {
// We can't construct a ParseIntError, so we have to pattern match.
assert!(matches!(
parse_pos_nonzero("not a number"),
Err(ParsePosNonzeroError::ParseInt(_))
));
}
#[test]
fn test_negative() {
assert_eq!(
parse_pos_nonzero("-555"),
Err(ParsePosNonzeroError::Creation(CreationError::Negative))
);
}
#[test]
fn test_zero() {
assert_eq!(
parse_pos_nonzero("0"),
Err(ParsePosNonzeroError::Creation(CreationError::Zero))
);
}
#[test]
fn test_positive() {
let x = PositiveNonzeroInteger::new(42);
assert!(x.is_ok());
assert_eq!(parse_pos_nonzero("42"), Ok(x.unwrap()));
}
}

117
exercises/error_handling/errorsn.rs
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@ -1,117 +0,0 @@
// errorsn.rs
// This is a bigger error exercise than the previous ones!
// You can do it! :)
//
// Edit the `read_and_validate` function ONLY. Don't create any Errors
// that do not already exist.
//
// So many things could go wrong!
//
// - Reading from stdin could produce an io::Error
// - Parsing the input could produce a num::ParseIntError
// - Validating the input could produce a CreationError (defined below)
//
// How can we lump these errors into one general error? That is, what
// type goes where the question marks are, and how do we return
// that type from the body of read_and_validate?
//
// Execute `rustlings hint errorsn` for hints :)
// I AM NOT DONE
use std::error;
use std::fmt;
use std::io;
// PositiveNonzeroInteger is a struct defined below the tests.
fn read_and_validate(b: &mut dyn io::BufRead) -> Result<PositiveNonzeroInteger, ???> {
let mut line = String::new();
b.read_line(&mut line);
let num: i64 = line.trim().parse();
let answer = PositiveNonzeroInteger::new(num);
answer
}
//
// Nothing below this needs to be modified
//
// This is a test helper function that turns a &str into a BufReader.
fn test_with_str(s: &str) -> Result<PositiveNonzeroInteger, Box<dyn error::Error>> {
let mut b = io::BufReader::new(s.as_bytes());
read_and_validate(&mut b)
}
#[test]
fn test_success() {
let x = test_with_str("42\n");
assert_eq!(PositiveNonzeroInteger(42), x.unwrap());
}
#[test]
fn test_not_num() {
let x = test_with_str("eleven billion\n");
assert!(x.is_err());
}
#[test]
fn test_non_positive() {
let x = test_with_str("-40\n");
assert!(x.is_err());
}
#[test]
fn test_ioerror() {
struct Broken;
impl io::Read for Broken {
fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> {
Err(io::Error::new(io::ErrorKind::BrokenPipe, "uh-oh!"))
}
}
let mut b = io::BufReader::new(Broken);
assert!(read_and_validate(&mut b).is_err());
assert_eq!("uh-oh!", read_and_validate(&mut b).unwrap_err().to_string());
}
#[derive(PartialEq, Debug)]
struct PositiveNonzeroInteger(u64);
impl PositiveNonzeroInteger {
fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
if value == 0 {
Err(CreationError::Zero)
} else if value < 0 {
Err(CreationError::Negative)
} else {
Ok(PositiveNonzeroInteger(value as u64))
}
}
}
#[test]
fn test_positive_nonzero_integer_creation() {
assert!(PositiveNonzeroInteger::new(10).is_ok());
assert_eq!(
Err(CreationError::Negative),
PositiveNonzeroInteger::new(-10)
);
assert_eq!(Err(CreationError::Zero), PositiveNonzeroInteger::new(0));
}
#[derive(PartialEq, Debug)]
enum CreationError {
Negative,
Zero,
}
impl fmt::Display for CreationError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let description = match *self {
CreationError::Negative => "Number is negative",
CreationError::Zero => "Number is zero",
};
f.write_str(description)
}
}
impl error::Error for CreationError {}

82
info.toml
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@ -490,42 +490,61 @@ hint = """
If other functions can return a `Result`, why shouldn't `main`?"""
[[exercises]]
name = "errorsn"
path = "exercises/error_handling/errorsn.rs"
name = "errors4"
path = "exercises/error_handling/errors4.rs"
mode = "test"
hint = """
First hint: To figure out what type should go where the ??? is, take a look
at the test helper function `test_with_str`, since it returns whatever
`read_and_validate` returns and `test_with_str` has its signature fully
specified.
Next hint: There are three places in `read_and_validate` that we call a
function that returns a `Result` (that is, the functions might fail).
Apply the `?` operator on those calls so that we return immediately from
`read_and_validate` if those function calls fail.
`PositiveNonzeroInteger::new` is always creating a new instance and returning an `Ok` result.
It should be doing some checking, returning an `Err` result if those checks fail, and only
returning an `Ok` result if those checks determine that everything is... okay :)"""
[[exercises]]
name = "errors5"
path = "exercises/error_handling/errors5.rs"
mode = "compile"
hint = """
Hint: There are two different possible `Result` types produced within
`main()`, which are propagated using `?` operators. How do we declare a
return type from `main()` that allows both?
Another hint: under the hood, the `?` operator calls `From::from`
on the error value to convert it to a boxed trait object, a Box<dyn error::Error>,
which is polymorphic-- that means that lots of different kinds of errors
can be returned from the same function because all errors act the same
since they all implement the `error::Error` trait.
on the error value to convert it to a boxed trait object, a
`Box<dyn error::Error>`, which is polymorphic-- that means that lots of
different kinds of errors can be returned from the same function because
all errors act the same since they all implement the `error::Error` trait.
Check out this section of the book:
https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
This exercise uses some concepts that we won't get to until later in the
course, like `Box` and the `From` trait. It's not important to understand
them in detail right now, but you can read ahead if you like.
Another another hint: Note that because the `?` operator returns
the *unwrapped* value in the `Ok` case, if we want to return a `Result` from
`read_and_validate` for *its* success case, we'll have to rewrap a value
that we got from the return value of a `?`ed call in an `Ok`-- this will
look like `Ok(something)`.
Read more about boxing errors:
https://doc.rust-lang.org/stable/rust-by-example/error/multiple_error_types/boxing_errors.html
Read more about using the `?` operator with boxed errors:
https://doc.rust-lang.org/stable/rust-by-example/error/multiple_error_types/reenter_question_mark.html
"""
Another another another hint: `Result`s must be "used", that is, you'll
get a warning if you don't handle a `Result` that you get in your
function. Read more about that in the `std::result` module docs:
https://doc.rust-lang.org/std/result/#results-must-be-used"""
[[exercises]]
name = "errors6"
path = "exercises/error_handling/errors6.rs"
mode = "test"
hint = """
This exercise uses a completed version of `PositiveNonzeroInteger` from
errors4.
Below the line that TODO asks you to change, there is an example of using
the `map_err()` method on a `Result` to transform one type of error into
another. Try using something similar on the `Result` from `parse()`. You
might use the `?` operator to return early from the function, or you might
use a `match` expression, or maybe there's another way!
You can create another function inside `impl ParsePosNonzeroError` to use
with `map_err()`.
Read more about `map_err()` in the `std::result` documentation:
https://doc.rust-lang.org/std/result/enum.Result.html#method.map_err"""
# Generics
@ -561,7 +580,7 @@ ReportCard struct generic, but also the correct property - you will need to chan
of the struct slightly too...you can do it!
"""
# OPTIONS / RESULTS
# OPTIONS
[[exercises]]
name = "option1"
@ -603,15 +622,6 @@ statement. How can this be avoided? The compiler shows the correction
needed. After making the correction as suggested by the compiler, do
read: https://doc.rust-lang.org/std/keyword.ref.html"""
[[exercises]]
name = "result1"
path = "exercises/error_handling/result1.rs"
mode = "test"
hint = """
`PositiveNonzeroInteger::new` is always creating a new instance and returning an `Ok` result.
It should be doing some checking, returning an `Err` result if those checks fail, and only
returning an `Ok` result if those checks determine that everything is... okay :)"""
# TRAITS
[[exercises]]
@ -920,7 +930,7 @@ hint = """
Follow the steps provided right before the `TryFrom` implementation.
You can also use the example at https://doc.rust-lang.org/std/convert/trait.TryFrom.html
You might want to look back at the exercise errorsn (or its hints) to remind
You might want to look back at the exercise errors5 (or its hints) to remind
yourself about how `Box<dyn Error>` works.
If you're trying to return a string as an error, note that neither `str`