I often find this pattern in Haskell code:
options :: MVar OptionRecord
options = unsafePerformIO $ newEmptyMVar
...
doSomething :: Foo -> Bar
doSomething = unsafePerformIO $ do
opt <- readMVar options
doSomething' where ...
Basically, one has a record of options or something similar, that is initially set at the program's beginning. As the programmer is lazy, he doesn't want to carry the options record all over the program. He defines an MVar to keep it - defined by an ugly use of unsafePerformIO. The programmer ensures, that the state is set only once and before any operation has taken place. Now each part of the program has to use unsafePerformIO again, just to extract the options.
In my opinion, such a variable is considered pragmatically pure (don't beat me). Is there a library that abstracts this concept away and ensures that the variable is set only once, i.e. that no call is done before that initialization and that one doesn't have to write unsafeFireZeMissilesAndMakeYourCodeUglyAnd DisgustingBecauseOfThisLongFunctionName
If you are using MVar for holding settings or something similar, why don't you try reader monad?
foo :: ReaderT OptionRecord IO ()
foo = do
options <- ask
fireMissiles
main = runReaderT foo (OptionRecord "foo")
(And regular Reader if you don't require IO :P)
Those who would trade essential referential transparency for a little temporary convenience deserve neither purity nor convenience.
This is a bad idea. The code that you're finding this in is bad code.*
There's no way to fully wrap this pattern up safely, because it is not a safe pattern. Do not do this in your code. Do not look for a safe way to do this. There is not a safe way to do this. Put the unsafePerformIO down on the floor, slowly, and back away from the console...
*There are legitimate reasons that people do use top level MVars, but those reasons have to do with bindings to foreign code for the most part, or a few other things where the alternative is very messy. In those instances, as far as I know, however, the top level MVars are not accessed from behind unsafePerformIO.
Use implicit parameters. They're slightly less heavyweight than making every function have Reader or ReaderT in its type. You do have to change the type signatures of your functions, but I think such a change can be scripted. (Would make a nice feature for a Haskell IDE.)
There is an important reason for not using this pattern. As far as I know, in
options :: MVar OptionRecord
options = unsafePerformIO $ newEmptyMVar
Haskell gives no guarantees that options will be evaluated only once. Since the result of option is a pure value, it can be memoized and reused, but it can also be recomputed for every call (i.e. inlined) and the meaning of the program must not change (contrary to your case).
If you still decide to use this pattern, be sure to add {-# NOINLINE options #-}, otherwise it might get inlined and your program will fail! (And by this we're getting out of the guarantees given by the language and the type system and relying solely on the implementation of a particular compiler.)
This topic has been widely discussed and possible solutions are nicely summarized on Haskell Wiki in Top level mutable state. Currently it's not possible to safely abstract this pattern without some additional compiler support.
I often find this pattern in Haskell code:
Read different code.
As the programmer is lazy, he doesn't want to carry the options record all over the program. He defines an MVar to keep it - defined by an ugly use of unsafePerformIO. The programmer ensures, that the state is set only once and before any operation has taken place. Now each part of the program has to use unsafePerformIO again, just to extract the options.
Sounds like literally exactly what the reader monad accomplishes, except that the reader monad does it in a safe way. Instead of accommodating your own laziness, just write actual good code.
来源：https://stackoverflow.com/questions/6076129/a-way-to-avoid-a-common-use-of-unsafeperformio