(First time using a mailing list; I hope this message ends up in the correct 
thread)

This is a topic that comes up regularly on the Swift evolution mailing list and 
off it.
After reading through all the respective threads again, there seem to be the 
following two camps:

Arguments made for crashing when accessing a non-existent index: 
- fast
- shows bugs quickly 

Arguments made in favor of returning an optional by default: 
- safe (as in "doesn't crash")
- similar to what other modern languages do
- what an unexperienced Swift developer would expect

All are valid arguments, but for different use cases. 
In my opinion, the biggest problem is that there's no indication that 
subscripting can crash on the default array. Alternative subscripts for bounded 
access wouldn't solve this either.

Maybe Swift should have two different array classes: A fast, fast-failing 
"UnsafeArray" and a default safe "Array". This would prevent unexpected crashes 
for new Swift programmers while still providing a faster alternative for those 
who do low-level stuff. The name "UnsafeArray" would clearly communicate that 
this class should be handled with care.

- Andreas
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