Uniplate

From HaskellWiki

I am pleased to announce Uniplate (formerly known as Play), a library for boilerplate removal requiring only Haskell 98 (for normal use) and optionally multi-parameter type classes (for more advanced features). This library has been tested with Yhc, Hugs and GHC.

The Uniplate library can do some of the same things as [[Scrap Your Boilerplate]] (SYB), and has functions that can be used in a similar manner to everywhere and everything.

1 Links

• Project home page: http://www-users.cs.york.ac.uk/~ndm/uniplate/
• Hackage release: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uniplate-1.0
• Haddock docs: http://www.cs.york.ac.uk/fp/haddock/uniplate/
• Short Manual: http://www.cs.york.ac.uk/fp/darcs/uniplate/uniplate.htm
• Draft Paper: http://www-users.cs.york.ac.uk/~ndm/downloads/draft-uniplate-15_jun_2007.pdf

The manual contains a basic overview of some bits of the library, the paper goes into more detail, but is intended to still be a readable introduction to the library.

2 Example

data Expr  =  Add  Expr  Expr  | Val  Int
           |  Sub  Expr  Expr  | Var  String
           |  Mul  Expr  Expr  | Neg  Expr
           |  Div  Expr  Expr

We can extract all the variables using the function:

variables :: Expr -> [String]
variables (Var  x  ) = [x]
variables (Val  x  ) = []
variables (Neg  x  ) = variables x
variables (Add  x y) = variables x ++ variables y
variables (Sub  x y) = variables x ++ variables y
variables (Mul  x y) = variables x ++ variables y
variables (Div  x y) = variables x ++ variables y

Or we can use the Uniplate library:

variables :: Expr -> [String]
variables x = [y | Var y <- universe x]

3 Thanks to

Colin Runciman, Bjorn Bringert, Jules Bean, Eric Mertens, Tom Shackell and Stefan O'Rear for various help and feedback.

4 Trailer

The Uniplate library has been used in Yhc, Catch, Hoogle and other projects. Please direct any follow-up discussion to haskell-cafe.

Thanks Neil

Category: Libraries