Data.MLens.Ref

Data type for reference lenses

type Ref m a = MLens m () a

Note that references lenses can be composed with lenses. For example, if

r :: Ref m (a,b)

then

fstLens . r :: Ref m a

Reference laws for pure references:

• (readRef r) has no side effect
• (readRef r >>= writeRef r) === (return ())
• (writeRef r a >> readRef r) === (return a)
• (writeRef r a >> writeRef r a') === (writeRef r a')
• Reference laws need not be preserved by composition, but they should be preserved if a pure lens is composed from the left.

These first four laws are equivalent to the get-no-effect, set-get, get-set and set-set laws for monadic lenses.

Reference operations

readRef :: Monad m => MLens m () a -> m a

writeRef :: Monad m => Ref m a -> a -> m ()

modRef :: Monad m => Ref m a -> (a -> a) -> m ()

Some impure IO referenceses

fileRef :: FilePath -> IO (Ref IO String)

Using fileRef is safe if the file is not used concurrently.

fileRef_ :: FilePath -> IO (Ref IO (Maybe String))

Note that if you write Nothing, the file is deleted.

logConsoleLens :: Show a => MLens IO a a

logConsoleLens logs elementary get and set operations.

Note that with the current representation of MLens, every set operation involves a get operation.

Auxiliary definitions

logMLens :: Monad m => (a -> m ()) -> (a -> m ()) -> MLens m a a

logFile :: FilePath -> IO (String -> IO ())