Haskell token "#"

Question

What does the # token mean and how is it used?

newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))

I came across this here and wonder where this type is actually defined or weather this is some syntax of a internal type that is ultimately defined in C?

Answer 1

将#类型标记为拆箱类型。该#是传统的，但需要一个扩展（MagicHash），使其有效的语法。同样，(# ... #)是未装箱元组的语法。