# Uniplate

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.

## 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
- 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://ndmitchell.com/downloads/paper-uniform_boilerplate_and_list_processing-30_sep_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.

## Example

Given an `Int`

expression data type:

```
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]
```

## Thanks to

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

## 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