Control Flow

Let Statement

A let statement can be used to define a function within the scope of another function.

For example, let's look at the following code:

is_large_area width height =
    let area = width * height
        area > 100

When this function is called, it will first assign the product of the width and height to an area function, then call that area function to check if it's greater than 100, returning a boolean value.

This function body can also be written as width * height > 100, but using a let statement can make the code somewhat more readable.

It can also be written in one line using a -> symbol:

is_large_area width height = let area = width * height -> area > 100

If Statement

An if statement can be used to return one of two different values depending on if a condition is true or false.

For example, let's look at the following code:

square_or_circle is_square =
    if is_square
        SQUARE
    else
        CIRCLE

This will return a square when is_square is true and a circle when is_square is false.

It can also be written in one line using the -> symbol:

square_or_circle is_square = if is_square -> SQUARE else -> CIRCLE

To check for multiple conditions, you can chain if statements together like so:

shape n =
    if n > 0
        SQUARE
    else if n < 0
        CIRCLE
    else
        TRIANGLE

In this example, the shape function will return a square when n is 1 or higher, a circle when n is -1 or lower, and a triangle when n is 0.

Match Statement

A match statement can be used to compare a value to multiple patterns and return a value when a pattern is matched.

For example, let's look at the following code:

shape n =
    match n
        0
            SQUARE
        1
            CIRCLE
        2
            TRIANGLE
        _
            EMPTY

This will return a square when n is 0, a circle when n is 1, a triangle when n is 2, and default to an empty shape when n is any other value.

It can also be written in one line using the -> symbol:

shape n = match n -> 0 -> SQUARE; 1 -> CIRCLE; 2 -> TRIANGLE; _ -> EMPTY

You can also check for multiple patterns in a single case by separating the patterns with commas:

shape n =
    match n
        0,1,2 -> SQUARE
        3,4,5 -> CIRCLE
        6,7,8 -> TRIANGLE
        _ -> EMPTY

Loop

A loop can be used to generate a list of values of a defined amount.

For example, let's look at the following code:

squares n =
    loop n
        SQUARE

This will return a list of squares with a length of n. Say that we called this function with squares 3. This would return the list [SQUARE, SQUARE, SQUARE].

It can also be written in one line using the -> symbol:

squares n = loop n -> SQUARE

This particular function wouldn't be very helpful, because all of the squares would be drawn on top of each other. One thing loops are useful for is generating a list of randomized shapes.

For example, we can create a list of randomly rotated squares like so:

squares n =
    loop n
        r (rand * 360) SQUARE

To learn more about random functions, please visit the Randomness page.

For Loop

A for loop can be used to generate a list of values by iterating over an existing list or range.

For example, let's look at the following code:

add_to_range n amount =
    for i in 0..amount
        i + n

This will iterate over the range 0..amount, which is an exclusive range, meaning that if the amount is 3 the range will include [0, 1, 2]. An inclusive range can be represented with 0..=amount, which would include [0, 1, 2, 3].

Each value in the range is assigned to the i function. Then, for each i, it will add n and return it.

Say that we called this function with add_to_range 1 5. This will return the list [1, 2, 3, 4, 5], because it would iterate over the range 0 to 4 and add one to each value.

It can also be written in one line using the -> symbol:

add_to_range n amount = for i in 0..amount -> i + n