@hackage fclabels0.4.0

First class labels that act as bidirectional records fields. The labels are fully composable and can be used to get, set and modify part of datatypes in a consistent way. The label datatype, conveniently called :->, is an instance of the Category type class meaning it has a proper identity and composition. The library has support for automatically deriving labels from record selectors that start with an underscore. Labels can be used in a pure functional setting or be applied to mutable state in some state monad.

To illustrate this package take the following two example datatypes (somehow Haddock removes the braces):

data Person = Person {
    _name   :: String
  , _age    :: Int
  , _isMale :: Bool
  , _place  :: Place
  }

data Place = Place {
    _city
  , _country
  , _continent :: String
  }

Both are record datatypes with all record labels prefixed with an underscore. This underscore is an indication for our Template Haskell code to derive labels for these fields. Deriving labels can be done with this simple one-liner:

$(mkLabels [''Person, ''Place])

Label function will be generated, label type signatures will not. This is actually not that bad, by writing the signatures down yourself you will be able to give them documentation, something that would be hard otherwise. So, lets give functions a signature by hand:

name      :: Person :-> String
age       :: Person :-> Int
isMale    :: Person :-> Bool
place     :: Person :-> Place
city      :: Place :-> String
country   :: Place :-> String
continent :: Place :-> String

These type signatures look very similar to the function types for normal record labels, except that the additional colon indicates a true first class label. These labels can be used to get, set and modify the value and are fully composable.

Now let look at this example. This 71 old fellow, called Jan, is my neighbour and didn't mind using him as an example:

jan :: Person
jan = Person "Jan" 71 True (Place "Utrecht" "The Netherlands" "Europe")

When we want to be sure Jan is really as old as he claims we can use the get function for labels to get the age out as an integer:

hisAge :: Int
hisAge = get age jan

Consider he now wants to move to Amsterdam, what better place to spend your old days. Using composition when can change the city value deep inside the structure:

moveToAmsterdam :: Person -> Person
moveToAmsterdam = set (city . place) "Amsterdam"

moveToAmsterdam jan ==
 Person "Jan" 71 True (Place "Amsterdam" "The Netherlands" "Europe")

Composition is done using the dot operator which is part of the Control.Category module. Make sure to import this module and hide the default (.), id and mod function from the Prelude.

Now, because Jan is an old guy moving to another city is not a very easy task, this really takes a while. So it will probably take no less than two years before he will actually be settled. To reflect this change it might be useful to have a first class view on the Person data type that only reveals the age and city. This can be done by using a neat Applicative functor instance:

ageAndCity :: Person :-> (Int, String)
ageAndCity = Label $ (,) <$> fst `for` age <*> snd `for` (city . place)

Because the applicative type class on its own is not very capable of expressing bidirectional relations, which we need for our labels, the actual instance is defined for an internal helper structure called Point. Points are a bit more general than labels. As you can see above the Label constructor has to be used to convert a Point back into a Label. The for function must be used to indicate which partial destructor to use for which label in the applicative composition.

Now that we have an appropriate age+city view on the Person data type (which is itself a label again), we can use the mod function to make Jan move to Amsterdam over exactly two years:

moveToAmsterdamOverTwoYears :: Person -> Person
moveToAmsterdamOverTwoYears = mod ageAndCity (\(a, b) -> (a+2, "Amsterdam"))

moveToAmsterdamOverTwoYears jan ==
 Person "Jan" 73 True (Place "Amsterdam" "The Netherlands" "Europe")

This package also contains a lens data type that encodes bidirectional functions. Just like labels lenses can be composed to other lenses using the Control.Category type class. Lenses can be used to change the type of a label. The Iso type class, which can be seen as a bidirectional functor, can be used to apply lenses to labels. For example, when we want to threat the age of a person as a string we can do the following:

ageAsString :: Person :-> String
ageAsString :: (show <-> read) `iso` age

This library might look cryptic at first sight, but give it a try it is not that hard.