Yesterday, I uploaded the library `unsafely` to Hackage: http://hackage.haskell.org/package/unsafely This package provides you the functionality for access control for unsafe operations and instances. This purpose is somewhat similar to GHC's `NullaryTypeClasses`[^1] extension, but permits more flexible access control. With this package, you can tag functions and type-class instances as *unsafe* in type constraint. This library is useful when: * You want to restrict the access to *unsafe* operations by type constraint * You have to provide some *unsafe* type-instances for practical reasons. For example, when writing computer algebra system with type-classes, `Double` type doesn't even form a semi ring, but we need the instance `Semiring Double` if we want to combine the symbolic computations and the numerical methods. A simple example: ```haskell {-# LANGUAGE FlexibleContexts, FlexibleInstances, RankNTypes #-} {-# LANGUAGE UndecidableInstances #-} module Main where import Data.Constraint.Unsafely import Data.IORef import Data.Proxy import System.IO.Unsafe saferUnsafePerformIO :: Unsafely IO => IO a -> a saferUnsafePerformIO = unsafePerformIO global :: Unsafely IO => IORef Int global = saferUnsafePerformIO $ newIORef 0 unsafelyIO :: (Unsafely IO => a) -> a unsafelyIO = unsafely (Proxy :: Proxy IO) main :: IO () main = do unsafelyIO $ readIORef global -- | uncommenting following line causes type-error! -- readIORef global return () ``` For more detail, please read Haddock[^2]. [^1]: https://ghc.haskell.org/trac/ghc/ticket/7642 [^2]: http://hackage.haskell.org/package/unsafely-0.1.0.0.1/docs/Data-Constraint-U... -- Hiromi ISHII konn.jinro@gmail.com