unsafePerformIO was there to make the indeterminate evaluation order visible, but isn't what causes that indeterminate evaluation order. You are right, though, that for 'safe' Haskell code the evaluation order shouldn't matter, although it is sort of a matter of convention; 'head' is unsafe, but is presumably now permanently stuck in the language, while on the other hand perhaps one could write C++ without exceptions, but that wouldn't be 'normal' C++ code.

Sure, my point was that, as far as I understand, even Haskell code that performed IO would have a well-determined order, if it was doing normal IO; whereas C++ code that does something as simple and safe looking as foo(i++, i) may produce different results in different runs (it was even UB before C++17, and still us in C; and even other common idioms, such as function chaining, had undefined execution order).

Yes and no. The order IO is performed is well-defined (that's the purpose of the IO monad), but the evaluation order is not (apart from ordering arising from data dependency), and this is the case with or without unsafePerformIO. You just won't get any different behaviour with safe Haskell code, and Haskell is very much on the safe side of things, so even though the order of evaluation may be indeterminate, you would never notice it (maybe a timing attack is possible?). But this is admittedly hair-splitting.