1> var a: [Int] = [0,1,2,3] a: [Int] = 4 values { [0] = 0 [1] = 1 [2] = 2 [3] = 3 } 2> var b: [String] = a as! [String] Could not cast value of type 'Swift.Int' (0x1013e0430) to 'Swift.String' (0x1013e36f8). 2017-10-06 11:35:18.511301+0900 repl_swift[28561:24355268] Could not cast value of type 'Swift.Int' (0x1013e0430) to 'Swift.String' (0x1013e36f8). b: [String] = <extracting data from value failed> Execution interrupted. Enter code to recover and continue. Enter LLDB commands to investigate (type :help for assistance.) Process 28561 stopped * thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGABRT frame #0: 0x00007fffc4cf5d42 libsystem_kernel.dylib`__pthread_kill + 10 libsystem_kernel.dylib`__pthread_kill: -> 0x7fffc4cf5d42 <+10>: jae 0x7fffc4cf5d4c ; <+20> 0x7fffc4cf5d44 <+12>: movq %rax, %rdi 0x7fffc4cf5d47 <+15>: jmp 0x7fffc4ceecaf ; cerror_nocancel 0x7fffc4cf5d4c <+20>: retq Target 0: (repl_swift) stopped.