Syntax

Monkey has a C-like syntax that should feel familiar to programmers coming from languages like JavaScript, C, or Go. This section covers the fundamental syntax elements of the Monkey programming language.

Identifiers

Identifiers in Monkey must start with a letter, followed by any combination of letters, digits, or underscores:

let myVariable = 10;
let counter1 = 0;

Keywords

Monkey has the following reserved keywords:

• let - Variable binding
• fn - Function definition
• if - Conditional statement
• else - Alternative branch for conditionals
• return - Return statement
• true - Boolean true literal
• false - Boolean false literal

Operators

Arithmetic Operators

let a = 5 + 3;    // Addition
let b = 10 - 4;   // Subtraction
let c = 6 * 7;    // Multiplication
let d = 15 / 3;   // Division

Comparison Operators

let equal = 5 == 5;        // Equality
let notEqual = 5 != 3;     // Inequality
let less = 3 < 5;          // Less than
let greater = 7 > 4;       // Greater than

Logical Operators

let negation = !true;      // Logical NOT

Statements vs Expressions

Monkey distinguishes between statements and expressions:

Statements

• let statements for variable binding
• return statements for returning values
• Expression statements (expressions used as statements)

Expressions

• Literals (numbers, strings, booleans)
• Identifiers
• Prefix expressions (!, -)
• Infix expressions (+, -, *, /, ==, !=, <, >)
• Function calls
• If expressions
• Function literals

Semicolons

Semicolons are generally optional in Monkey, but they can be used to explicitly terminate statements:

let x = 5
let y = 10;

Both lines above are valid. Semicolons are required when you want to put multiple statements on the same line:

let x = 5; let y = 10;