Add some exercises about functions!

functions/functions1.rs

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+// Make me compile! Scroll down for hints :)
+
+fn main() {
+    call_me();
+}
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+// This main function is calling a function that it expects to exist, but the
+// function doesn't exist. It expects this function to have the name `call_me`.
+// It expects this function to not take any arguments and not return a value.
+// Sounds a lot like `main`, doesn't it?

functions/functions2.rs

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+// Make me compile! Scroll down for hints :)
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+fn main() {
+    call_me(3);
+}
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+fn call_me(num) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}
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+// Rust requires that all parts of a function's signature have type annotations,
+// but `call_me` is missing the type annotation of `num`.

functions/functions3.rs

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+// Make me compile! Scroll down for hints :)
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+fn main() {
+    call_me();
+}
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+fn call_me(num: i32) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}
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+// This time, the function *declaration* is okay, but there's something wrong
+// with the place where we're calling the function.

functions/functions4.rs

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+// Make me compile! Scroll down for hints :)
+
+// This store is having a sale where if the price is an even number, you get
+// 10 (money unit) off, but if it's an odd number, it's 3 (money unit) less.
+
+fn main() {
+    let original_price = 51;
+    println!("Your sale price is {}", sale_price(original_price));
+}
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+fn sale_price(price: i32) -> {
+    if is_even(price) {
+        price - 10
+    } else {
+        price - 3
+    }
+}
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+fn is_even(num: i32) -> bool {
+    num % 2 == 0
+}
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+// The error message points to line 10 and says it expects a type after the
+// `->`. This is where the function's return type should be-- take a look at
+// the `is_even` function for an example!