How to return Error Message from assertion in Kotlin

Question

I am getting comfortable in Kotlin after switching from Java and could not find out how to use Kotlin's assert function in conjunction with an error message.

Sounds simple, I just need something similar to Java's

assert count > 5 : "value too small"

I tried

assert(count > 5, "value too small")

However, the second argument needs to be () -> Any. How to achieve that?

Answer 1

assert's message parameter is not a String, but a function returning a String. This is so because otherwise, since assert is a normal Kotlin function, its arguments would be evaluated every time which would lead to unnecessary overhead (and sometimes change in semantics) of calculating the message string in case that string is a complex expression.

To pass a function argument, use the lambda syntax. The last argument which is a lambda may be left out of the parentheses:

assert(count > 5) { "value too small" }