If Then Else Expressions | CodeSignal Learn

If Then Else Expressions | CodeSignal Learn

Introduction: Making Decisions in Code

Welcome to Unit 6 of our 7-unit journey into Haskell! In our previous lesson, we learned how to keep complex calculations organized using local let and where bindings . Now, it is time to teach our programs how to make choices. In the real world, we make decisions constantly based on specific conditions. For example, if a number is below zero, we might want to flip its sign to make it positive; otherwise, we leave it exactly as it is. To handle simple, two-way decisions like this, Haskell provides the if/then/else syntax. It relies on a Bool (true or false) condition to decide which path your code should take. In this lesson, we will learn how to write these choices cleanly and safely.

Expressions and the Mandatory else

In many programming languages, an if statement is an action — it simply tells the computer to do something. However, in Haskell, if/then/else is an expression . This is a very important distinction. Just like the math expression 5 + 3 evaluates to the single value 8, a Haskell if expression evaluates to a single piece of data. Let us build a function step by step to calculate the absolute (positive) value of an integer and see this in action. First, we write our type signature and define the function named absValue with a single input parameter, n . absValue :: Int -> Int absValue n = Next, we introduce our condition using the if keyword. We want to check if n is less than zero. absValue :: Int -> Int absValue n = if n < 0 Now, we must tell Haskell what value this expression should become if the condition is true. We do this using the then keyword. If n is less than zero, we will use the built-in negate function (which flips the sign of a number) to return a positive value. absValue :: Int -> Int absValue n = if n < 0 then negate n Finally, we must provide the else branch. What happens if the condition is false? Because if/then/else is an expression that yields a value, the else branch is strictly mandatory. Haskell refuses to compile an if without an else because it always needs to know what value to hand back. If the number is not less than zero, we just return n . absValue :: Int -> Int absValue n = if n < 0 then negate n else n Our function is now complete. If n is -7, the expression evaluates to 7 . If n is 5, the expression evaluates to 5 .

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The Golden Rule: Matching Types

Because if/then/else hands back a single value, there is a second strict rule you must follow: the value returned by the then branch and the value returned by the else branch must be of the exact same data type. Let us build another function to see why this matters. We want to write a function named fareLabel that takes a person's age as an Int and returns a String ticket label. We start with the signature and condition. Haskell fareLabel :: Int -> String fareLabel age = if age < 18 Next, we add the then branch for ages under eighteen. Haskell fareLabel :: Int -> String fareLabel age = if age < 18 then "child" Finally, we add our else branch for adults. Haskell fareLabel :: Int -> String fareLabel age = if age < 18 then "child" else "adult" Notice that both "child" and "adult" are of type String . This perfectly follows the golden rule. Imagine if we tried to write if age < 18 then "child" else 100 . Haskell's strict compiler would throw an error right away. Because Haskell needs to know the single, definitive return type of every expression, it cannot allow an expression that might return a String half the time and an Int the other half. This rule keeps our code safe and predictable.

Putting It Together in main

Now that we have written our two decision-making functions, let us bring them together inside our main block to test them. As a quick reminder from our earlier units, we use the do keyword to sequence multiple actions, print to display numeric data, and putStrLn to display strings. First, we set up our main block. Haskell main :: IO () main = do Next, let us call our absValue function with a negative number and print the result. We use parentheses around -7 so Haskell knows it is a negative number, not a subtraction operation. Haskell main :: IO () main = do print (absValue (-7)) Now, let us test our fareLabel function. We will call it twice with different ages and use putStrLn to print the string results. Haskell main :: IO () main = do print (absValue (-7)) putStrLn (fareLabel 15) putStrLn (fareLabel 30) When we run this complete program, Haskell evaluates the conditions in our functions and prints the correct choices to the console. Here is the complete code containing all the steps we just built together: Haskell absValue :: Int -> Int absValue n = if n < 0 then negate n else n fareLabel :: Int -> String fareLabel age = if age < 18 then "child" else "adult" main :: IO () main = do print (absValue (-7)) putStrLn (fareLabel 15) putStrLn (fareLabel 30) And here is the output you will see in the console: text 7 child adult