"Variable 'xxx' was never mutated; consider changing to 'let' constant" ERROR

Question

I have the following problem. I use this code below and I get the issue

"Variable 'characteristic' was never mutated; consider changing to 'let' constant"

for var characteristic:CBCharacteristic in service.characteristics ?? [] {     print(str)     _selectedPeripheral!.writeValue(str.dataUsingEncoding(NSUTF8StringEncoding)!, forCharacteristic: characteristic, type: CBCharacteristicWriteType.WithoutResponse) } 

When I change to "let", there's an Error:

'let' pattern cannot appear nested in an already immutable context

Why does it recommend me the change and afterwards mark it as an error?

Answer 1

You just need to remove var, making your code:

for characteristic in service.characteristics ?? [] {     print(str)     _selectedPeripheral!.writeValue(str.dataUsingEncoding(NSUTF8StringEncoding)!, forCharacteristic: characteristic, type: CBCharacteristicWriteType.WithoutResponse) } 

because characteristic is immutable by default.