data-lens-ixset-0.1.2: A Lens for IxSet




ixLens :: (Indexable a, Typeable a, Typeable k, Ord a) => k -> Lens (IxSet a) (Maybe a)Source

Focus on a key in an indexed set.

Given an IxSet of people:

 people = fromList [ Person (FirstName "Edward A.") (LastName "Kmett")
                   , Person (FirstName "Simon") (LastName "P. Jones")

We can now work with indices using lenses and fix Simon's last name:

 people' = ixLens (FirstName "Simon") ^%= fmap (lastName ^= LastName "Peyton-Jones") $ people

Perhaps more commonly you're working with an IxSet from inside a state monad such as with acid-state. In that case usage is even easier:

 changeLastName = ixLens (FirstName "Simon") %= fmap (lastName ^= LastName "Peyton-Jones")

Here's the missing boilerplate:

 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE TemplateHaskell #-}

 import Data.Data
 import Data.IxSet
 import Data.Lens
 import Data.Lens.Template

 data Person = Person { _firstName :: FirstName
                      , _lastName  :: LastName
                      } deriving (Show, Eq, Ord, Data, Typeable)

 mkLens ''Person

 newtype FirstName = FirstName String
   deriving (Show, Eq, Ord, Data, Typeable)

 newtype LastName = LastName String
   deriving (Show, Eq, Ord, Data, Typeable)

 instance Indexable Person where
   empty = ixSet [ ixGen (Proxy :: Proxy FirstName)
                 , ixGen (Proxy :: Proxy LastName)