Conditionals

Table of contents
  1. Your first conditional
  2. The structure
  3. Comparison operators
    1. Equality
    2. Not equal
    3. Greater than and less than
    4. Greater than or equal, less than or equal
  4. Practical examples
    1. Temperature warning
    2. Discount eligibility
    3. Pass/fail grade
  5. Combining with where bindings
  6. Logical operators
    1. AND - both must be true
    2. OR - at least one must be true
    3. NOT - flip the condition
  7. Nested conditionals
  8. Comparing strings
  9. Checking membership
  10. Real-world examples
    1. Email filtering
    2. Pricing logic
    3. Access control
    4. Form validation
  11. Important notes
  12. Try it yourself!

So far, every expression you’ve written evaluates to a single value. But what if you need different results depending on some condition? That’s where if-then-else comes in!

Your first conditional 

if true then "yes" else "no"

This evaluates to "yes" because the condition is true.

if false then "yes" else "no"

This evaluates to "no" because the condition is false.

The structure

Every if expression has three parts:

if CONDITION then VALUE_IF_TRUE else VALUE_IF_FALSE

All three parts are required. Melbi doesn’t allow “dangling ifs” - you must always specify what happens in both cases.

Comparison operators

To make real decisions, you need to compare values:

Equality 

if 5 == 5 then "equal" else "not equal"

if 3 == 7 then "equal" else "not equal"

Not equal 

if 5 != 3 then "different" else "same"

Greater than and less than 

if 10 > 5 then "bigger" else "smaller or equal"

if 3 < 8 then "smaller" else "bigger or equal"

Greater than or equal, less than or equal 

if 5 >= 5 then "yes" else "no"

if 10 <= 8 then "yes" else "no"

Practical examples

Temperature warning 

if temperature > 30 then "It's hot!" else "Nice weather"
where { temperature = 35 }

Discount eligibility 

if purchase_amount >= 100 then "Free shipping!" else "Standard shipping"
where { purchase_amount = 150 }

Pass/fail grade 

if score >= 70 then "Pass" else "Fail"
where { score = 85 }

Combining with where bindings

You can use where bindings to organize your conditional logic:

message where {
    age = 16,
    can_drive = age >= 16,
    message = if can_drive then "You can drive!" else "Too young to drive",
}

Or put the condition directly in the if:

if age >= 18 then "Adult" else "Minor"
where { age = 21 }

Logical operators

Combine multiple conditions with and, or, and not:

AND - both must be true 

if age >= 18 and has_license then "Can drive" else "Cannot drive"
where { age = 20, has_license = true }

OR - at least one must be true 

if is_weekend or is_holiday then "Day off!" else "Work day"
where { is_weekend = false, is_holiday = true }

NOT - flip the condition 

if not is_raining then "Go outside" else "Stay inside"
where { is_raining = false }

Nested conditionals

You can nest if expressions inside each other:

if age < 13 then "Child"
else if age < 20 then "Teenager"
else if age < 65 then "Adult"
else "Senior"
where { age = 35 }

Note: This is actually if ... then ... else (if ... then ... else (...)) - each else contains another if.

Comparing strings

You can compare strings too:

if name == "Alice" then "Hello Alice!" else "Hello stranger!"
where { name = "Alice" }

if status != "active" then "Account inactive" else "Welcome back!"
where { status = "suspended" }

Checking membership

Use in to check if something is contained:

if "error" in log_message then "Alert!" else "All good"
where { log_message = "error: connection timeout" }

if "@gmail.com" in email then "Gmail user" else "Other email"
where { email = "user@gmail.com" }

Use not in for the opposite:

if "spam" not in subject then "Show email" else "Move to spam"
where { subject = "Important meeting" }

Real-world examples

Email filtering 

if "urgent" in subject or sender == "boss@company.com"
then "Priority inbox"
else "Regular inbox"
where {
    subject = "Urgent: Project deadline",
    sender = "colleague@company.com",
}

Pricing logic 

final_price where {
    base_price = 50,
    is_member = true,
    quantity = 10,
    discount = if is_member and quantity >= 10 then 0.15 else 0.05,
    final_price = base_price * quantity * (1 - discount),
}

Access control 

if role == "admin" or (role == "moderator" and is_owner)
then "Full access"
else "Read only"
where {
    role = "moderator",
    is_owner = true,
}

Form validation 

if age >= 18 and email_valid and agreed_to_terms
then "Registration successful"
else "Please complete all requirements"
where {
    age = 25,
    email_valid = true,
    agreed_to_terms = true,
}

Important notes

  1. Both branches (then and else) must return the same type - you can’t have if x then 5 else "hello". This prevents type errors.

  2. The condition must be a boolean (true or false) - you can’t use numbers or strings directly as conditions.

  3. You must always have an else - Melbi requires you to handle both cases explicitly.

Try it yourself!

Write expressions that:

  1. Check if a number is positive, negative, or zero
  2. Determine shipping cost based on purchase amount
  3. Convert a numeric grade to a letter grade (A, B, C, D, F)
  4. Check if a password meets requirements (length, has numbers, etc.)
  5. Calculate parking fees based on hours parked and day of week

Congratulations! You’ve completed the Basics section. You now know how to:

  • Perform calculations
  • Use variables with where bindings
  • Work with text
  • Make decisions with conditionals

Next, you’ll learn about working with collections of data like arrays, records, and maps!