Why am I able to use my value constructor even though I don't export it?

问题

For practice, I'm implementing a queue data type in a module called "Queue". My data type is also called "Queue", as is its only value constructor:

module Queue (Queue, enq, emptyQueue) where

data Queue a = Queue {
  inbox  :: [a],
  outbox :: [a]
} deriving (Eq, Show)

emptyQueue :: Queue a
emptyQueue = Queue [] []

enq :: a -> Queue a -> Queue a
enq x (Queue inb out) = Queue (x:inb) out

-- other function definitions (irrelevant here)...

As far as I understand, because I wrote Queue, not Queue(..) or Queue(Queue) in the export statement, I don't expect the value constructor of my data type to be exported by the module. This is exactly what I want, for encapsulation purposes: users should not be able to use the value constructor directly; only emptyQueue, enq, and the other functions in my interface.

However (and the solution to my problem may be obvious to seasoned Haskellers), if I load my module in GHCi, I'm able to use the value constructor directly.

$ ghci Queue.hs
GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Queue            ( Queue.hs, interpreted )
Ok, modules loaded: Queue.

λ> Queue [1] [2] 
Queue {inbox = [1], outbox = [2]}

As stated above, this is undesirable. What am I doing wrong?

回答1:

You are doing nothing wrong. It's just that, for convenience, ghci ignores scoping rules on modules it loads.

If you want to see what would normally be available, you can

:m -Queue
:m +Queue

You can later return to the "everything is available" mode with

:m *Queue

See also What's really in scope at the prompt? in the official documentation.

来源：https://stackoverflow.com/questions/28116009/why-am-i-able-to-use-my-value-constructor-even-though-i-dont-export-it