The pez package

[Tags: bsd3, 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.


Properties

Versions0.0.1, 0.0.2, 0.0.3, 0.0.4, 0.1.0
Dependenciesbase (==4.*), fclabels (>=0.11.1.1 && <0.12), thrist (==0.2.*)
LicenseBSD3
CopyrightBrandon Simmons, 2011
AuthorBrandon Simmons
Maintainerbrandon.m.simmons@gmail.com
StabilityExperimental
CategoryData
Home pagehttp://coder.bsimmons.name/blog/2011/04/pez-zipper-library-released/
Upload dateThu Apr 21 15:10:08 UTC 2011
Uploaded byBrandonSimmons
Downloads301 total (19 in last 30 days)

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