language-deep-dive5 min

Contracts with require

A require statement is a precondition — a rule that must be true when a function is called. Instead of burying defensive checks deep in the function body, require states the contract at the top where every reader sees it immediately.

require enforces the rules a function needs to work correctly:

functions:\n    number divide(number a, number b)\n        require b != 0.0\n        return a / b\n\n    string repeat(string text, integer times)\n        require times > 0\n        require text.length() > 0\n        string result = ""\n        iterate i in 0 to times - 1\n            result = result + text\n        return result\n\nstart:\n    print(divide(10.0, 4.0).toString())\n    print(repeat("ab", 3))

2.5\nababab

require b != 0.0 states that divide requires b to be non-zero. If someone calls divide with 0.0, an error is signaled before the division — no cryptic runtime crash. Multiple require statements are allowed and checked in order.

require makes the valid input range immediately visible:

functions:\n    string grade(integer score)\n        require score >= 0\n        require score <= 100\n        if score >= 90\n            return "A"\n        if score >= 80\n            return "B"\n        if score >= 70\n            return "C"\n        if score >= 60\n            return "D"\n        return "F"\n\nstart:\n    print(grade(95))\n    print(grade(73))\n    print(grade(55))

A\nC\nF

Anyone reading grade sees immediately: this function works with scores 0 to 100. The require statements are the function's contract — they document the expected inputs and enforce them. Code that passes invalid values fails at the require line, not somewhere inside the logic.

Quick recap

require condition signals an error if the condition is false
Place require statements at the very top of the function, before any logic
Multiple require lines are allowed — each is checked in order
require is documentation AND enforcement — the contract is visible to every reader