Where Bindings

Table of contents
  1. Your first where binding
  2. How it works
  3. Multiple bindings
  4. Using bindings in other bindings
  5. Multi-line format
  6. Practical examples
    1. Distance calculation
    2. Compound interest
    3. Temperature conversion
  7. Why use where bindings?
  8. Important rules
  9. Try it yourself!

In the last lesson, you learned how to write simple calculations. But what if your expression gets more complex? Repeating values and sub-calculations makes code hard to read and maintain.

That’s where where bindings come in - they let you give names to values and reuse them!

Your first where binding

Let’s calculate a total price with tax:

price * (1 + tax) where { price = 100, tax = 0.08 }

This reads naturally: “price times (1 plus tax), where price is 100 and tax is 0.08”

The result is 108.0.

How it works

A where binding has two parts:

  1. The main expression (before where) - this is what you’re calculating
  2. The bindings (after where in curly braces) - these define the values you’re using

Think of it like solving a math problem where you’re given some values first, then use them in a formula.

Multiple bindings

You can define as many names as you need, separated by commas:

a + b + c where { a = 5, b = 10, c = 15 }

The names are available to use anywhere after they’re defined.

Using bindings in other bindings

Here’s where it gets powerful - you can use earlier bindings to calculate later ones:

total where {
    price = 100,
    tax_rate = 0.08,
    tax = price * tax_rate,
    total = price + tax,
}

Notice how tax uses price and tax_rate, then total uses both price and tax. Each binding can reference any binding defined before it.

Multi-line format

For longer expressions, you can write them across multiple lines for readability:

final_price where {
    base_price = 299.99,
    discount = 0.15,
    discounted = base_price * (1 - discount),
    tax_rate = 0.0725,
    tax = discounted * tax_rate,
    final_price = discounted + tax,
}

Try clicking the ▶️ button and see how each step builds on the previous one!

Practical examples

Distance calculation 

distance where {
    x1 = 0,
    y1 = 0,
    x2 = 3,
    y2 = 4,
    dx = x2 - x1,
    dy = y2 - y1,
    distance = (dx ^ 2 + dy ^ 2) ^ 0.5,
}

This calculates the distance between two points: (0,0) and (3,4).

Compound interest 

amount where {
    principal = 1000,
    rate = 0.05,
    years = 10,
    amount = principal * ((1 + rate) ^ years),
}

Calculate how much $1000 grows at 5% interest over 10 years.

Temperature conversion 

fahrenheit where {
    celsius = 25,
    fahrenheit = (celsius * 9.0 / 5.0) + 32.0,
}

Convert 25°C to Fahrenheit.

Why use where bindings?

  1. Clarity - Name your values so your expression reads like English
  2. Reusability - Calculate something once, use it multiple times
  3. Organization - Break complex calculations into understandable steps
  4. No repetition - Don’t write price * 1.08 three times, write price once

Important rules

  1. Names must start with a lowercase letter: price ✓, Price ✗
  2. Names can contain letters, numbers, and underscores: tax_rate_2024 ✓
  3. You can’t redefine a name - each name appears once
  4. You can only use names that were defined before (or to the left of) where you use them

Try it yourself!

Try writing these expressions with where bindings:

  1. Calculate the area of a circle given its radius (use 3.14159 for π)
  2. Figure out how many days until your next birthday
  3. Calculate a 20% tip on a $45.50 meal
  4. Convert miles per hour to meters per second

In the next lesson, you’ll learn how to work with text!