Re: [問題] '#'的意思

[joshs (Josh Ko)]

※ 引述《etwas (i'm only dust)》之銘言：
: 各位版友好
: 想請問在source code中 '(#', '#)'是什麼意思? 例如:
: instance Monad (ST s) where
:     return x = ST (\ s -> (# s, x #))
: 這在哪裡的文件有提到呢?
: 謝謝

這是 GHC 的 unboxed tuples:

  Unboxed tuples are used for functions that need to return
  multiple values, but they avoid the heap allocation normally
  associated with using fully-fledged tuples. When an unboxed
  tuple is returned, the components are put directly into
  registers or on the stack; the unboxed tuple itself does not
  have a composite representation. Many of the primitive
  operations listed in primops.txt.pp return unboxed tuples.
  In particular, the IO and ST monads use unboxed tuples to
  avoid unnecessary allocation during sequences of operations.

以上摘自

  http://www.haskell.org/ghc/docs/latest/html/users_guide/primitives.html

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