Update Exercises Directory Names to Reflect Order

exercises/13_error_handling/README.md

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+# Error handling
+
+Most errors aren’t serious enough to require the program to stop entirely.
+Sometimes, when a function fails, it’s for a reason that you can easily interpret and respond to.
+For example, if you try to open a file and that operation fails because the file doesn’t exist, you might want to create the file instead of terminating the process.
+
+## Further information
+
+- [Error Handling](https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html)
+- [Generics](https://doc.rust-lang.org/book/ch10-01-syntax.html)
+- [Result](https://doc.rust-lang.org/rust-by-example/error/result.html)
+- [Boxing errors](https://doc.rust-lang.org/rust-by-example/error/multiple_error_types/boxing_errors.html)

exercises/13_error_handling/errors1.rs

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+// errors1.rs
+//
+// This function refuses to generate text to be printed on a nametag if you pass
+// it an empty string. It'd be nicer if it explained what the problem was,
+// instead of just sometimes returning `None`. Thankfully, Rust has a similar
+// construct to `Option` that can be used to express error conditions. Let's use
+// it!
+//
+// Execute `rustlings hint errors1` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+pub fn generate_nametag_text(name: String) -> Option<String> {
+    if name.is_empty() {
+        // Empty names aren't allowed.
+        None
+    } else {
+        Some(format!("Hi! My name is {}", name))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn generates_nametag_text_for_a_nonempty_name() {
+        assert_eq!(
+            generate_nametag_text("Beyoncé".into()),
+            Ok("Hi! My name is Beyoncé".into())
+        );
+    }
+
+    #[test]
+    fn explains_why_generating_nametag_text_fails() {
+        assert_eq!(
+            generate_nametag_text("".into()),
+            // Don't change this line
+            Err("`name` was empty; it must be nonempty.".into())
+        );
+    }
+}

exercises/13_error_handling/errors2.rs

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+// errors2.rs
+//
+// Say we're writing a game where you can buy items with tokens. All items cost
+// 5 tokens, and whenever you purchase items there is a processing fee of 1
+// token. A player of the game will type in how many items they want to buy, and
+// the `total_cost` function will calculate the total cost of the items. Since
+// the player typed in the quantity, though, we get it as a string-- and they
+// might have typed anything, not just numbers!
+//
+// Right now, this function isn't handling the error case at all (and isn't
+// handling the success case properly either). What we want to do is: if we call
+// the `total_cost` function on a string that is not a number, that function
+// will return a `ParseIntError`, and in that case, we want to immediately
+// return that error from our function and not try to multiply and add.
+//
+// There are at least two ways to implement this that are both correct-- but one
+// is a lot shorter!
+//
+// Execute `rustlings hint errors2` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+use std::num::ParseIntError;
+
+pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
+    let processing_fee = 1;
+    let cost_per_item = 5;
+    let qty = item_quantity.parse::<i32>();
+
+    Ok(qty * cost_per_item + processing_fee)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn item_quantity_is_a_valid_number() {
+        assert_eq!(total_cost("34"), Ok(171));
+    }
+
+    #[test]
+    fn item_quantity_is_an_invalid_number() {
+        assert_eq!(
+            total_cost("beep boop").unwrap_err().to_string(),
+            "invalid digit found in string"
+        );
+    }
+}

exercises/13_error_handling/errors3.rs

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+// errors3.rs
+//
+// This is a program that is trying to use a completed version of the
+// `total_cost` function from the previous exercise. It's not working though!
+// Why not? What should we do to fix it?
+//
+// Execute `rustlings hint errors3` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+use std::num::ParseIntError;
+
+fn main() {
+    let mut tokens = 100;
+    let pretend_user_input = "8";
+
+    let cost = total_cost(pretend_user_input)?;
+
+    if cost > tokens {
+        println!("You can't afford that many!");
+    } else {
+        tokens -= cost;
+        println!("You now have {} tokens.", tokens);
+    }
+}
+
+pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
+    let processing_fee = 1;
+    let cost_per_item = 5;
+    let qty = item_quantity.parse::<i32>()?;
+
+    Ok(qty * cost_per_item + processing_fee)
+}

exercises/13_error_handling/errors4.rs

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+// errors4.rs
+//
+// Execute `rustlings hint errors4` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+#[derive(PartialEq, Debug)]
+struct PositiveNonzeroInteger(u64);
+
+#[derive(PartialEq, Debug)]
+enum CreationError {
+    Negative,
+    Zero,
+}
+
+impl PositiveNonzeroInteger {
+    fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
+        // Hmm... Why is this always returning an Ok value?
+        Ok(PositiveNonzeroInteger(value as u64))
+    }
+}
+
+#[test]
+fn test_creation() {
+    assert!(PositiveNonzeroInteger::new(10).is_ok());
+    assert_eq!(
+        Err(CreationError::Negative),
+        PositiveNonzeroInteger::new(-10)
+    );
+    assert_eq!(Err(CreationError::Zero), PositiveNonzeroInteger::new(0));
+}

exercises/13_error_handling/errors5.rs

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+// errors5.rs
+//
+// This program uses an altered version of the code from errors4.
+//
+// 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. For now, think
+// of the `Box<dyn ???>` type as an "I want anything that does ???" type, which,
+// given Rust's usual standards for runtime safety, should strike you as
+// somewhat lenient!
+//
+// In short, this particular use case for boxes is for when you want to own a
+// value and you care only that it is a type which implements a particular
+// trait. To do so, The Box is declared as of type Box<dyn Trait> where Trait is
+// the trait the compiler looks for on any value used in that context. For this
+// exercise, that context is the potential errors which can be returned in a
+// Result.
+//
+// What can we use to describe both errors? In other words, is there a trait
+// which both errors implement?
+//
+// Execute `rustlings hint errors5` or use the `hint` watch subcommand for a
+// hint.
+
+// 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<(), Box<dyn ???>> {
+    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 {}

exercises/13_error_handling/errors6.rs

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+// 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.
+//
+// Execute `rustlings hint errors6` or use the `hint` watch subcommand for a
+// hint.
+
+// 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 from_parseint...
+}
+
+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()));
+    }
+}