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

The # marks the type as an unboxed type. The # is conventional, but requires an extension (MagicHash) to make it valid syntax. Likewise, (# ... #) is the syntax for an unboxed tuple.