This modules provides a convienient way to access and update the elements of a structure. It is very similar to Data.Accessors, but a bit more generic and has fewer dependencies. I particularly like how cleanly it handles nested structures in state monads. runSTLense is also a very useful function.

A brief tutorial to get started:

To create a lense, you can use fromGetSet (although usually you would just derive them using templat haskell and deriveLenses from Data.Lenses.Template):

 lense = fromGetSet getField setField

The lense has type:

 lense :: (MonadState r m) => StateT a m b -> m b

Where r is the type of the record, and a is the type of the field, (b can be any type you choose, more on that latter). Though it may help to think of it as:

 lense :: State a b -> State r b

Which is not entirely accurate, but emphasises how the lense works. You can think of it as "pass in an action that operates on the field, and you get an action that operates on the record". So say we pass in get (with a more specific type for clarity):

 get :: State a a

 lense get :: State r a

We get out a state monad that we can run on our record to fetch our field.

 fieldValue = lense get `evalState` record

This module has a special function fetch that does this:

 fieldValue = record `fetch` lense

You can also pass in put to get back an action that updates the field.

 put :: a -> State a ()

 lense (put someValue) :: State r ()

Now we have a state monad that we can run on our record to update our field.

 updatedRecord = lense (put someValue) `execState` record

This module has a special function update that does this:

 updatedRecord = (record `update` lense) someValue

To aid in clarity and to deal with the actual types of the lenses this module provides execIn, evalFrom, and runOn to be used in place of execState, evalState, and runState. Also note that execIn, evalFrom, and runOn have their parameters fliped from their state counterparts. There is nothing magical about these functions, they are just a little more handy than their state counterparts.

The lenses are especially convienient if you have nested structures. Lense composition is just function composition.

 data Point = Point {
                 x_ :: Float,
                 y_ :: Float
                 }
    deriving (Show)

$( deriveLenses ''Point )

 data Triangle = Triangle {
                 pa_ :: Point,
                 pb_ :: Point,
                 pc_ :: Point
                 }
    deriving (Show)

$( deriveLenses ''Triangle )

 a_y :: (MonadState Triangle m) => StateT Float (StateT Point m) b -> m b
 a_y = pa . y

a_y is now a lense that can operate on the y coordinate of point "a" inside a triangle. We can use a_y to fetch the coordinate or update it, on whatever triangle we choose.

 someTriangle = Triangle (Point 5 3) (Point 0 1) (Point 10 6)

 ayValue = someTriangle `fetch` a_y
 -- ayValue == 3

 updatedTriangle = (someTriangle `update` a_y) 7
 -- updatedTriangle == Triangle (Point 5 7) (Point 0 1) (Point 10 6)

Or we could apply our lense to an action and pass it into execIn

 (someTriangle `update` a_y) 7 == execIn someTriangle (a_y (put 7))

We can also chain actions together:

 a_x :: (MonadState Triangle m) => StateT Float (StateT Point m) b -> m b
 a_x = pa . x
 c_y :: (MonadState Triangle m) => StateT Float (StateT Point m) b -> m b
 c_y = pc . y

 updatedTriangle = execIn someTriangle $ a_y (put 7) >> a_x (put 1) >> c_y (put 9)
 -- updatedTriangle == Triangle (Point 1 7) (Point 0 1) (Point 10 9)

What if we wanted to put the value of c_y into a_x? Can do!

  updatedTriangle = execIn someTriangle $ do
    cy <- c_y get
    a_x $ put cy
  -- updatedTriangle == Triangle (Point 6 3) (Point 0 1) (Point 10 6)

Or if the order really bugs you, you can use the $% operator (taken from Data.Accessors.Basic, it really should be in a standard lib)

  updatedTriangle = execIn someTriangle $ do
    cy <- get $% c_y
    put cy $% a_x
  -- updatedTriangle == Triangle (Point 6 3) (Point 0 1) (Point 10 6)

Or you can use the $= operator:

  updatedTriangle = execIn someTriangle $ do
    cy <- c_y get
    a_x $= cy

Or more concisely:

  updatedTriangle = execIn someTriangle $ (c_y get >>= a_x . put)

Or say we want to put the value of c_y into a_x, but want to throw an error if c_y is zero. We can do that as well!

  updatedTriangle :: Either String Triangle
  updatedTriangle = execInT someTriangle $ do
    cy <- c_y get
    when (cy == 0) $ throwError "Something bad happend"
    a_x $ put cy
  -- updatedTriangle == Right $ Triangle (Point 6 3) (Point 0 1) (Point 10 6)
  -- if cy had equaled 0 then we would have gotten this:
  -- updatedTriangle == Left "Something bad happend"

Note that execInT = flip execStateT.

Yay for monad transformers!

This module has one last feature that allows you to convert a function that fetches data from a structure to a function that modifies it! For an example see the documentation for runSTLense.

One final note: Due to the generality of the lenses you might end up accidentally running into the monomorphism restriction. So if get a type error like:

    Couldn't match expected type `SomeMonad SomeStructureType'
           against inferred type `Control.Monad.Identity.Identity SomeStructureType'

and nothing appears to be wrong with your code, try turning the restriction off with -XNoMonomorphismRestriction and see if it goes away. If it does then you probably need to add some explicit type signatures somewhere.

I whipped out this documentation in a hurry, so if you spot any errors, or think I should explain something better, please let me know. This library is hosted on github (click on the Contents link above and you should see the Homepage link) so it should be very easy to forked it and send patches to me. Also since this module is new I'm open to radical modifications if you have a good suggestion, so suggest away! :)