iOS8 Swift Development: let Var difference discussion

localhost:~ hejiasheng$ xcrun swiftwelcome to swift!  Type:help for assistance.  1> var a:inta:int = 0  2> let b:int<repl>:2:5: Error: "Let" declarations require an initializer expression Let B:int    ^  2> let b = 5b:int = 5  3> b = 6  4> println (6) 6

5> println (b)
6

Many places say that the let variable cannot be changed, and I test the discovery can be changed.

But it needs to be initialized at the time of definition.

Playground case, will be error. can be negative.