pez: A Potentially-Excellent Zipper library

PEZ is a generic zipper library. It uses lenses from the fclabels package to reference a "location" to move to in the zipper. The zipper is restricted to types in the Typeable class, allowing the user to "move up" through complex data structures such as mutually-recursive types.

Both the Typeable class and fclabels lenses can be derived in GHC, making it easy for the programmer to use a zipper with a minimum of boilerplate.

First import the library, which brings in the Typeable and fclabels modules. You will also want to enable a few extensions:

-- Put these in a LANGUAGE pragma:
-- TemplateHaskell, DeriveDataTypeable, TypeOperators
module Main where

import Data.Typeable.Zipper

Create a datatype, deriving an instance of the Typeable class, and generate a lens using functions from fclabels:

data Tree a = Node {
    _leftNode :: Tree a
  , _val      :: a
  , _rightNode :: Tree a }
  | Nil
  deriving (Typeable,Show)

$(mkLabelsNoTypes [''Tree])

Now we can go crazy using Tree in a Zipper:

treeBCD = Node (Node Nil 'b' Nil) 'c' (Node Nil 'd' Nil)

descendLeft :: Zipper1 (Tree a) -> Zipper1 (Tree a)
descendLeft z = case (viewf z) of
                     Nil -> z
                     _   -> descendLeft $ moveTo leftNode z

insertLeftmost :: a -> Tree a -> Tree a
insertLeftmost x = close . setL focus x . descendLeft . zipper

treeABCD = insertLeftmost 'a' treeBCD

Because of the flexibility of fclabels, this zipper library can be used to express moving about in reversible computations simply by defining such a lens, for instance:

stringRep :: (Show b, Read b) => b :-> String
stringRep = lens show (const . read)

Please send any feature requests or bug reports along.

Modules

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• pez-0.0.4.tar.gz [browse] (Cabal source package)
• Package description (as included in the package)

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• BrandonSimmons

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