feat: added closures exercises in closure directory

Closures as a separate exercises appeared to be missing, so I added some. Have placed them in the watch order after functions, which seemed reasonable.

exercises/closures/README.md

@@ -0,0 +1,17 @@
+# Closures
+
+Closures are a common programming concept found in many languages, but are primarily
+associated with 'functional' programming styles. Closure are anonymous functions
+(i.e. unlike regular functions, they don't have a name) that may access some variables
+from their enclosing scope (they 'close' over their scope, capturing variables).
+
+Closures are often used as arguments to higher order functions (functions which 
+take other functions as parameters), e.g. map(), filter(), reduce() et al on iterators,
+but aren't restricted to these cases.
+
+Rust's ownership model means that we might have to do something extra when declaring
+closures which other languages don't require.
+
+## Further information
+
+- [Closures: Anonymous Functions that Can Capture Their Environment](https://doc.rust-lang.org/book/ch13-01-closures.html)

exercises/closures/closures1.rs

@@ -0,0 +1,21 @@
+// closures1.rs
+// Closures are anonymous functions, but they can be assigned to a variable
+// so that they can be used later on
+// make me compile and pass
+
+// Execute `rustlings hint closures1` for hints!
+
+// I AM NOT DONE
+
+fn main() {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    # [test]
+    fn test_closure() {
+        let closure = ;
+        assert_eq!("I'm a closure",closure());
+    }
+}

exercises/closures/closures2.rs

@@ -0,0 +1,24 @@
+// closure2.rs
+// What is the difference between a closure and a function? Can you get this code
+// to compile?
+
+// Execute `rustlings hint closures2` for hints!
+
+// I AM NOT DONE
+
+fn output(closure) {
+    println!("What am I?");
+    closure();
+}
+
+fn main() {
+
+    fn actual() {
+        println!("I'm actually a function")
+    }
+
+    let actual_fn = actual;
+    let closure = || println!("I'm a closure");
+    output(closure);
+    output(actual_fn);
+}

exercises/closures/closures3.rs

@@ -0,0 +1,24 @@
+// closure3.rs
+// What is the difference between a closure and a function, part II? Can you get this code
+// to compile?
+
+// Execute `rustlings hint closures3` for hints!
+
+// I AM NOT DONE
+
+fn main() {
+    let my_name = String::from("Lim Lady");
+
+    fn actual() {
+        println!("my name is: {}", my_name);
+    }
+
+    let actual_fn = actual;
+    let closure = || {
+        println!("my name is: {}", my_name);
+    };
+
+    println!("Who am I?");
+    actual_fn();
+    closure();
+}

exercises/closures/closures4.rs

@@ -0,0 +1,34 @@
+// closure4.rs
+// Where are you going to use closures? Where the context defines the code
+// you want some other code to invoke. So, unlike functions or methods, closures
+// are for when the code is not generally useful, or attached to a
+// type, but is specific to the context you are defining it in. Closures can
+// define parameters, just like functions, but these are defined by the context
+// it is intended to run in, where the actual arguments will be supplied.
+// https://doc.rust-lang.org/stable/std/primitive.slice.html#method.sort_by
+
+// Execute `rustlings hint closures4` for hints!
+
+// I AM NOT DONE
+
+fn alphabetize(list: &mut Vec<&str>) {
+    list.sort_by();
+}
+
+fn main() {
+    let mut list = vec!("Oliver","Tarquinn","Bertrude");
+    println!("before {:?}",list);
+    alphabetize(& mut list);
+    println!("after {:?}",list);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    # [test]
+    fn test_alphabetize() {
+        let mut list = vec!("Oliver","Tarquinn","Bertrude");
+        assert_eq!(vec!("Bertrude","Oliver","Tarquinn"),alphabetize(&mut list));
+    }
+}

exercises/closures/closures5.rs

@@ -0,0 +1,44 @@
+// closure5.rs
+// These are some fairly common uses of closures on iterator methods
+// https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.map
+// https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.fold
+// my original version used 'reduce()' instead of 'fold()' but why would that
+// be unsuitable for a sum function in Rust (specifically)?
+
+// Execute `rustlings hint closures5` for hints!
+
+// I AM NOT DONE
+
+fn count_letters(animals: &Vec<&str>) -> Vec<usize>{
+    // the compiler should help you figure out what signature to use
+    // after you annotate it with one that fails.
+    let count_closure = ;
+    animals.iter().map(count_closure).collect()
+}
+
+fn sum_letters(animals: &Vec<&str>) -> usize {
+    let sum_closure =
+    let animals = count_letters(animals);
+    // pay close attention to the where clause in the function signature
+    // https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.fold
+    animals.iter().fold(0, sum_closure)
+}
+
+fn main() {
+    let animals= vec!["cat","fish","horse"];
+    println!("animals: {:?}",animals);
+    println!("length: {:?}", count_letters(&animals));
+    println!("total: {:?}", sum_letters(&animals));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    # [test]
+    fn test_() {
+        let animals= vec!["cat","fish","horse"];
+        assert_eq!(vec![3,4,5], count_letters(&animals));
+        assert_eq!(12, sum_letters(&animals));
+    }
+}

exercises/closures/mod.rs

@@ -0,0 +1,5 @@
+mod closures1;
+mod closures2;
+mod closures3;
+mod closures4;
+mod closures5;

exercises/collections/mod.rs

@@ -1,4 +1 @@
-mod hashmap1;
-mod hashmap2;
-mod vec1;
-mod vec2;
+mod closures1;

exercises/mod.rs

@@ -25,3 +25,4 @@ mod threads;
 mod traits;
 mod variables;
 mod lifetimes;
+mod closures;

info.toml

@@ -136,6 +136,50 @@ They are not the same. There are two solutions:
 1. Add a `return` ahead of `num * num;`
 2. remove `;`, make it to be `num * num`"""
 
+# CLOSURES
+
+[[exercises]]
+name = "closures1"
+path = "exercises/closures/closures1.rs"
+mode = "test"
+hint = """
+Take a look at this section of the book: https://doc.rust-lang.org/book/ch13-01-closures.html"""
+
+[[exercises]]
+name = "closures2"
+path = "exercises/closures/closures2.rs"
+mode = "compile"
+hint = """
+A bit taxing, but these should help:
+https://doc.rust-lang.org/rust-by-example/fn/closures/input_parameters.html
+https://doc.rust-lang.org/rust-by-example/fn/closures/anonymity.html
+https://doc.rust-lang.org/rust-by-example/fn/closures/input_functions.html"""
+
+[[exercises]]
+name = "closures3"
+path = "exercises/closures/closures3.rs"
+mode = "compile"
+hint = """
+The compiler has got you covered with this. After all, why IS it called a closure?"""
+
+[[exercises]]
+name = "closures4"
+path = "exercises/closures/closures4.rs"
+mode = "test"
+hint = """
+Don't try to actually implement any comparison other than using cmp:
+https://doc.rust-lang.org/stable/std/cmp/trait.Ord.html#tymethod.cmp
+e.g. x.cmp(y)"""
+
+[[exercises]]
+name = "closures5"
+path = "exercises/closures/closures5.rs"
+mode = "test"
+hint = """
+Honestly this was hard to write. Getting the signatures of closures right can be tough.
+Pay close attention to the documentation linked to in the exercise and which type
+parameters are the same in the function signatures."""
+
 # IF
 
 [[exercises]]