This package provides first class labels that can act as bidirectional record fields. The labels can be derived automatically using Template Haskell which means you don't have to write any boilerplate yourself. The labels are implemented as lenses and are fully composable. Labels can be used to get, set and modify parts of a datatype in a consistent way.

The lens datatype, conveniently called :->, is an instance of the Control.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.

To illustrate this package, let's take the following two example datatypes.

{-# LANGUAGE TemplateHaskell, TypeOperators #-}
import Control.Category
import Data.Label
import Prelude hiding ((.), id)

data Person = Person
  { _name   :: String
  , _age    :: Int
  , _place  :: Place
  } deriving Show

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

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

mkLabels [''Person, ''Place]

For all labels a lens will created.

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

jan :: Person
jan = Person "Jan" 71 (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 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 we can change the city value deep inside the structure:

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

And now:

ghci> moveToAmsterdam jan
Person "Jan" 71 (Place "Amsterdam" "The Netherlands" "Europe")

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

Now, because Jan is an old guy, moving to another city is not a very easy task, this really takes a while. 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 datatype that only reveals the age and city. This can be done by using a neat Applicative functor instance:

import Control.Applicative

(fstL, sndL) = $(getLabel ''(,))

ageAndCity :: Person :-> (Int, String)
ageAndCity = point $
  (,) <$> fstL >- age
      <*> sndL >- city . place

Because the applicative type class on its own is not capable of expressing bidirectional relations, which we need for our lenses, the actual instance is defined for an internal helper structure called Point. Points are a more general than lenses. As you can see above, the point function has to be used to convert a Point back into a Lens. The (>-) operator is used to indicate which partial destructor to use per arm of the applicative composition.

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

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

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

This package contains an isomorphisms datatype that encodes bidirectional functions, or better bidirectional categories. Just like lenses, isomorphisms can be composed using the Category type class. Isomorphisms can be used to change the type of a lens. Every isomorphism can be lifted into a lens.

For example, when we want to treat the age of a person as a string we can do the following:

ageAsString :: Person :-> String
ageAsString = iso (Iso show read) . age

Template Haskell functions for automatically generating labels for algebraic datatypes, newtypes and GADTs. There are two basic modes of label generation, the mkLabels family of functions create labels (and optionally type signatures) in scope as top level funtions, the getLabel family of funtions create labels as expressions that can be named and typed manually.

In the case of multi-constructor datatypes some fields might not always be available and the derived labels will be partial. Partial labels are provided with an additional type context that forces them to be only usable in the Partial or Failing context.

More derivation functions can be found in Data.Label.Derive.