language-deep-dive7 min

Functions in Depth

Functions in Clean Language are strictly typed — the compiler knows exactly what each function accepts and returns. This makes composition safe: when you call one function's output as another's input, the compiler verifies the types match.

Functions call other functions freely — build complex behavior from simple pieces:

functions:\n    integer clamp(integer value, integer min_val, integer max_val)\n        if value < min_val\n            return min_val\n        if value > max_val\n            return max_val\n        return value\n\n    string progress_bar(integer percent)\n        integer clamped = clamp(percent, 0, 100)\n        integer bars = clamped / 10\n        string filled = ""\n        iterate i in 0 to bars - 1\n            filled = filled + "#"\n        string empty = ""\n        iterate i in 0 to (10 - bars) - 1\n            empty = empty + "-"\n        return "[" + filled + empty + "] " + clamped.toString() + "%"\n\nstart:\n    print(progress_bar(30))\n    print(progress_bar(75))\n    print(progress_bar(110))

[###-------] 30%\n[#######---] 75%\n[##########] 100%

progress_bar calls clamp to enforce safe bounds, then builds the bar string. The typed parameters mean calling clamp with a string argument is a compile error. Multiple returns in different branches are fine — whichever branch executes returns from the function.

Functions that return boolean answer a yes/no question — perfect for if conditions:

functions:\n    boolean is_valid_email(string email)\n        if email.length() < 5\n            return false\n        if email.contains("@") == false\n            return false\n        if email.contains(".") == false\n            return false\n        return true\n\n    string check_email(string email)\n        if is_valid_email(email)\n            return email + " looks valid"\n        return email + " is not a valid email"\n\nstart:\n    print(check_email("alice@example.com"))\n    print(check_email("notanemail"))\n    print(check_email("short@x"))

alice@example.com looks valid\nnotanemail is not a valid email\nshort@x is not a valid email

is_valid_email returns boolean, so check_email can use it directly as an if condition. This is Clean Language's idiom for validation: extract the check into a boolean function, then call it where needed. The return type makes the function's intent obvious.

Quick recap

Functions call other functions freely — compose behavior from simple, testable pieces
Multiple return statements in different branches are allowed
Boolean functions answer yes/no questions and work directly as if conditions
Typed parameters are enforced at compile time — wrong argument types are caught early