> But a language that checked this property statically would give an error before execution even started. Like C or C++ does. But you are right - I shouldn't

> But a language that checked this property statically would give an error before execution even started. Like C or C++ does. But you are right - I shouldn't use 'in principle'. It was rather a decorative idiom here, because it is true only for simple functions like map or filter or whatever you take from a standard prelude. What I mean is that we are not talking about run-time in this context. Yes, one could pass by reference (or even by value) some crap at run-time, but this is a quite different problem. Machine code has this problem too.