# Functions This tutorial covers:

• Defining values
• Calling values Before starting, you should be familiar with: At the end, you should be able to:

• Describe the difference between a function parameter and a function argument
• Describe a function's return value
• Create a function with a single parameter
• Create a function with multiple parameters
• Provide arguments to a function
• Call a function from the interpreter

## Introduction

Jack and Rosa want to play a board game. They decide that who gets to go first should be determined by the result of a coin toss.

So far we have the name of our game `WhoGoesFirst`, a type that can describe who the first player is `Player`, a type for capturing a coin toss result `TossResult`, and a defined TossResult value `coin`. Now we need a way to determine a FirstPlayer value using a TossResult value. Since BoGL is largely a functional language we will do this by creating a function that takes a TossResult value as input and returns a FirstPlayer value.

Functions inside of BoGL are used to output, or return, some sort of value. The value that is returned from a function is often determined by the inputs that are provided to it. To illustrate the idea of a function in BoGL, let's first take a look at the math function below.

f(x) = 2x

This function takes in a value (x) which it uses to compute another value (double whatever value x is). In computer science terminology, x is what we call a parameter. If we were to assign our parameter, x, the value of 3, then the function would return the value 6. Shown below is how we can write this same function in BoGL. To create a function we need to write two things: A declaration(first line) and a definition(second line).

#### Function Declaration

The first part of a function is it's declaration(also known as a function signature). The declaration is where the types of the function's parameters and the type of the function's return value are declared. You can write a function declaration by first writing out the function's name. Just like value names, function names must start with a lowercase letter. In the above example our function is named f. After the name we write a colon `:`, which is then followed by the parameter types (there is just one parameter in the above example), followed by an arrow `->`, and lastly followed by the output type. The above function has an integer parameter and returns an integer.

#### Function Definition

The second part of a function is its definition. The definition is where all the logic goes that is used to determine the return value. The definition starts with the same function name that was written in the declaration, which is then followed by parentheses that contain the names of the function parameters separated by commas (there are no commas in the above example because there is only one parameter). Parameter names, like function names, must start with a lowercase letter. Following the ending parentheses is an equals sign `=`. After the equals sign is where the defining expression goes. You can think of the defining expression as the logic that determines the return value. The defining expression for the function above is `2 * x`. An expression is something that must be evaluated (like calculating 2 * 2) before it's value is determined. A function returns the result of evaluating it's defining expression. We will cover expressions in more detail in the next tutorial.

#### Calling a Function

To actually utilize this function we must call it. To call a function you must first type the function's name, followed by parenthesis `()`. Inside of those parenthesis you must put a value that matches the type of the function's parameter. We call this value a function argument. Try typing the function call `f(3)` into the interpreter below (right side textbox) and press the `enter` key. It should return the value 6.

game FunctionDemo

f : Int -> Int
f(x) = 2 * x

Lets now utilize what we have learned so far to make a function that takes a coin toss result as an input and outputs a first player. We will call this function determineFirstPlayer. It will have a TossResult as a parameter and return a Player. Here is it's function declaration:

Next we will create its defining expression. For now we will make this the value Rosa.

Together it will look like:

Try typing the function call `determineFirstPlayer(Heads)` into the interpreter below. It should return the value `Rosa`. You could also use `coin` as an argument for this function instead of `Heads`, since it's defined value is of type TossResult.

game WhoGoesFirst

type Player = {Jack, Rosa}

coin : TossResult

determineFirstPlayer : TossResult -> Player
determineFirstPlayer(coinSide) = Rosa

You may notice that the coinSide parameter is not being utilized, and that the output of this function will always be Rosa, no matter if the argument is Heads or Tails. In the if/else tutorial we will revisit this example and look at how we can replace the value Rosa with an expression that can evaluate to Jack or Rosa depending on the argument provided to the coinSide parameter.

## Functions with Multiple Parameters

A function can have multiple parameters by using tuples. The function shown below takes two numbers and returns the result of adding them together.

The number of parameters must correspond to the number of the function's parameter types.

You still need to specify parameter names when using a previously defined tuple type.