The Philosophy of Expressions: Why Is Everything a Value?

Let's be honest: have you ever been frustrated at having to write a program halfway around the fact that an if couldn't be placed inside an expression?

In most languages, if is a statement — it does something, but it doesn't produce a value. AHA! Lang takes a different path: almost everything is an expression that produces a value.

Expression vs Statement

// AHA! Lang: if/else is an expression —
// the last evaluated branch becomes the resulting value.

let x = 10;
let y = 20;

let result = if x > y {
    "x lebih besar"
} else {
    "y lebih besar"
};

print(result);

Because if produces a value, you can write it directly in an assignment. No two-step declaration, no variable being filled in inside a branch.

Why Is This Design Good?

1. Shorter, Clearer Programs

Compare a common pattern:

// common in statement-based languages:
let mut status = "default";
if score >= 7 {
    status = "lulus";
} else {
    status = "tidak lulus";
}

with the expression version:

let status = if score >= 7 { "lulus" } else { "tidak lulus" };

One expression, one value, no mutating state. Easier to read and harder to get wrong.

2. Composition

Values can be composed without limit:

let nilai = if a { 1 } else { 2 } * 10 + if b { 5 } else { 6 };

Control flow behaves like any other value — it can be operated on, passed around, and combined.

3. A Foundation for Functional Programming

This design is a natural gateway into a functional style: functions that map inputs to outputs without hidden side effects. This isn't a coincidence — it's a deliberate design decision, as you can see throughout the Language Tour.

Why Don't All Languages Do This?

Expression-oriented design usually requires a more disciplined compiler: every AST node must have a clear type and value. In AHA! Lang, it's the type system from release 1.4.0 that makes this philosophy possible — every expression is type-checked at compile time:

let a = 10;
let b = "teks";

// compile-time error: cannot compare Int with String
let hasil = if a == b { 1 } else { 2 };

Errors are caught before the program runs, not at runtime.

Conclusion

Everything a value, everything an expression — this is the heart of "Easy to read. Powerful to wield." If you want to dig into how the compiler brings this philosophy to life, follow our Course from the first lesson.