assignment 1~5: fixes

- assignment05/pascal.mlw: lowered the difficulty (one more invariant given)
- assignment02, 03: minor fixes & divide into sub-problems

src/assignments/assignment03/custom_operators.rs

@@ -0,0 +1,79 @@
+//! You will implement a number of custom operators.
+
+/// Custom option type.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum MyOption<T> {
+    /// Some value of type `T`.
+    MySome(T),
+    /// No value.
+    MyNone,
+}
+
+/// Maps an `MyOption<T>` to `MyOption<U>` by applying a function to a contained value.
+///
+/// # Examples
+///
+/// Converts an `MyOption<String>` into an `MyOption<usize>`, consuming the original:
+///
+/// ```
+/// use cs220::assignments::assignment03::{my_map, MyOption};
+///
+/// fn len(s: String) -> usize {
+///     s.len()
+/// }
+///
+/// assert_eq!(my_map(MyOption::MySome(String::from("Hello, World!")), len), MyOption::MySome(13));
+/// assert_eq!(my_map(MyOption::MyNone, len), MyOption::MyNone);
+/// ```
+pub fn my_map<T, U, F: FnOnce(T) -> U>(v: MyOption<T>, f: F) -> MyOption<U> {
+    todo!()
+}
+
+/// Returns `MyNone` if the option is `MyNone`, otherwise calls `f` with the wrapped value and returns the result.
+///
+/// Some languages call this operation flatmap.
+///
+/// # Examples
+///
+/// ```
+/// use cs220::assignments::assignment03::{MyOption, my_and_then};
+///
+/// fn pos_then_to_string(x: isize) -> MyOption<String> {
+///     if x > 0 {
+///         MyOption::MySome(x.to_string())
+///     } else {
+///         MyOption::MyNone
+///     }
+/// }
+///
+/// assert_eq!(my_and_then(MyOption::MySome(2), pos_then_to_string), MyOption::MySome(2.to_string()));
+/// assert_eq!(my_and_then(MyOption::MySome(-3), pos_then_to_string), MyOption::MyNone);
+/// assert_eq!(my_and_then(MyOption::MyNone, pos_then_to_string), MyOption::MyNone);
+/// ```
+pub fn my_and_then<T, U, F: FnOnce(T) -> MyOption<U>>(v: MyOption<T>, f: F) -> MyOption<U> {
+    todo!()
+}
+
+/// Custom operator: `option_op_or(v1, v2, f)`
+/// If neither `v1` nor `v2` is `Some`, returns `None`.
+/// If exactly one is `Some`, returns the same `Some` value.
+/// If both are `Some`, apply the values inside `Some` to `f` and wrap the resulting value inside `Some`.
+///
+/// # Examples
+///
+/// ```
+/// fn product(a: i32, b: i32) -> i32 {
+///     a * b
+/// }
+///
+/// assert_eq!(option_op_or(None, None, product), None);
+/// assert_eq!(option_op_or(Some(3), None, product), Some(3));
+/// assert_eq!(option_op_or(Some(3), Some(5), product), Some(15));
+/// ```
+pub fn my_option_op_or<T, F: FnOnce(T, T) -> T>(
+    v1: MyOption<T>,
+    v2: MyOption<T>,
+    f: F,
+) -> MyOption<T> {
+    todo!()
+}