generated at
UnboxedTuples
http://www.kotha.net/ghcguide_ja/7.6.2/primitives.html#unboxed-tuples
(# e_1, ..., e_n #) みたいなやつ
IOの実装とか覗くと見かける
hs
newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/primitives.html#glasgow-unboxed
https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/primitives.html#primitives