Conditionals

So far, your evaluator can compute with numbers, name values, and define functions. But it always does the same thing — there’s no way for it to choose.

Conditionals change that. With an if expression, the evaluator can look at a condition and decide which path to take.

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Comparisons

Before the evaluator can make decisions, it needs a way to ask questions. Expressions like [5, ">", 3] ask “is 5 greater than 3?” and produce a boolean — true or false.

These are the building blocks of decisions.

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The If Expression

Once you have booleans, you can use them to choose between two outcomes:

["if", [5, ">", 3], 10, 20]

This means: “if 5 is greater than 3, the result is 10, otherwise 20.”

The evaluator handles this in three steps:

1. Evaluate the condition
2. If the result is true, evaluate the consequent (the second expression)
3. If the result is false, evaluate the alternative (the third expression)

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What You’ll Do

You’ll extend your evaluator with:

• Comparison operators: <, >, ==, !=
• An if expression that picks a branch based on a condition

Let’s start by teaching the evaluator to compare.