Control Flow

Under Construction

The ability to run some code depending on whether a condition is true and to run some code repeatedly while a condition is true are basic building blocks in most programming languages. The most common constructs that let you control the flow of execution of Calyxium code are if expressions and loops.

If Expressions

An if expression allows you to branch your code depending on conditions. You provide a condition and then state, “If this condition is met, run this block of code. If the condition is not met, do not run this block of code.”

let number = 3 in

if number < 5 then print("condition was true") else print("condition was false")

All if expressions start with the keyword if, followed by a condition. In this case, the condition checks whether or not the variable number has a value less than 5. The block of code to execute if the condition is true follows immediately after the then keyword. Similarly, the block to execute if the condition is false follows the optional else keyword. These blocks of code are sometimes called arms.

Optionally, we can also include an else expression, which we chose to do here, to give the program an alternative block of code to execute should the condition evaluate to false. If you don’t provide an else expression and the condition is false, the program will just skip the if block and move on to the next bit of code.

Try running this code; you should see the following output:

condition was true

Let’s try changing the value of number to a value that makes the condition false to see what happens:

let number = 7 in

if number < 5 then print("condition was true") else print("condition was false")

Run the program again, and look at the output:

condition was false

It’s also worth noting that the condition in this code must be a bool. If the condition isn’t a bool, we’ll get an error. For example, try running the following code:

let number = 3 in

if number then print("condition was true") else print("condition was false")

The if condition evaluates to a value of 3 this time, and Calyxium throws an error:

.\main.cx: Type error in `if` expression condition:
  Expected: bool
  Found:    int

The error indicates that Calyxium expected a bool but got an integer. Unlike languages such as Ruby and JavaScript, Calyxium will not automatically try to convert non-Boolean types to a Boolean. You must be explicit and always provide if with a Boolean as its condition. If we want the if code block to run only when a number is not equal to 0, for example, we can change the if expression to the following:

let number = 3 in

if number != 0 then print("number was something other than zero")

Running this code will print number was something other than zero.

Note: if expressions in Calyxium are expression based, which means they evaluate to a value. You can assign their result to a variable or use them directly in larger expressions.