Data.MLens.Ref

Documentation

data Unit

The abstract data type Unit is isomorphic to (), but by making it abstract we can prevent using the same reference on both sides of lens composition, which would have surprising effects.

Instances

Eq Unit

Show Unit

Data type for reference lenses

type Ref m a = MLens m Unit 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 >> return ()) === (return ())
• (readRef r >>= writeRef r) === (return ())
• (writeRef r a >> readRef r) === (return a)
• (writeRef r a >> writeRef r a') === (writeRef r a')

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

Reference operations

readRef :: Monad m => Ref 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 ())