The pez package
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:
> -# LANGUAGE 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.
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|Versions||0.0.1, 0.0.2, 0.0.3, 0.0.4, 0.1.0|
|Dependencies||base (==4.*), fclabels (>=0.11.1.1 && <0.12), thrist (==0.2.*)|
|Copyright||Brandon Simmons, 2011|
|Upload date||Tue Apr 19 20:04:59 UTC 2011|
|Downloads||360 total (30 in last 30 days)|
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