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course/10-advanced_functions/exercises/1-higher-order/readme.md

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+# First Class and Higher Order Functions
+
+A programming language is said to have "first-class functions" when functions in that language are treated like any other variable. For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable.
+
+A function that returns a function or accepts a function as input is called a Higher-Order Function.
+
+Go supports [first-class](https://developer.mozilla.org/en-US/docs/Glossary/First-class_Function) and higher-order functions.  Another way to think of this is that a function is just another type -- just like `int`s and `string`s and `bool`s.
+
+For example, to accept a function as a parameter:
+
+```go
+func add(x, y int) int {
+  return x + y
+}
+
+func mul(x, y int) int {
+  return x * y
+}
+
+// aggregate applies the given math function to the first 3 inputs
+func aggregate(a, b, c int, arithmetic func(int, int) int) int {
+  return arithmetic(arithmetic(a, b), c)
+}
+
+func main(){
+  fmt.Println(aggregate(2,3,4, add))
+  // prints 9
+  fmt.Println(aggregate(2,3,4, mul))
+  // prints 24
+}
+```
+
+## Assignment
+
+Textio is launching a new email messaging product, "Mailio"!
+
+Fix the compile-time bug in the `getFormattedMessages` function. The function body is correct, but the function signature is not.