> If we can detect at compile time that a function is called more than once with the same arguments Just to emphasize, effect systems can let the compiler cha

> If we can detect at compile time that a function is called more than once with the same arguments Just to emphasize, effect systems can let the compiler chase down whole chains of functions calling functions and libraries and so on and know that at no point is there going to be anything non deterministic happening. Something you can't do, btw, if you follow the OP's advice and allow file reads.