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The primitive type `State#' represents the state of a state transformer.
It is parameterised on the desired type of state, which serves to keep
states from distinct threads distinct from one another. But the only
effect of this parameterisation is in the type system: all values of type
`State#' are represented in the same way. Indeed, they are all
represented by nothing at all! The code generator "knows" to generate no
code, and allocate no registers etc, for primitive states.
type State# s
The type `GHC.RealWorld' is truly opaque: there are no values defined
of this type, and no operations over it. It is "primitive" in that
sense -- but it is not unboxed! Its only role in life is to be
the type which distinguishes the `IO' state transformer.
data RealWorld
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