Design errors are a class of mistakes. Nobody can help you with design errors. Functional programming, immutability, static typing et al address other kinds of mistakes.

> when in fact most of the serious information leakages do not originate from such problems

We've already established that the parent post made a mistake in thinking FP is about safety against attackers / information leakage. Why are you still talking about this?

> because those languages are so insufferable that they induce design errors

[citation needed]

What's your actual experience with these languages?