@hackage bound0.1.3

The goal of this package is to make it as easy as possible to deal with name binding without forcing an awkward monadic style on the user. To that end we provide haskell 98 combinators for manipulating locally-nameless generalized de Bruijn terms, build over user-supplied term types. A generalized de Bruijn term is one where you can succ whole trees instead of just individual variables.

The approach was first elaborated in Bird and Patterson, "de Bruijn notation as a nested data type":

http://www.cs.uwyo.edu/~jlc/courses/5000_fall_08/debruijn_as_nested_datatype.pdf

However, the combinators they used required higher rank types. Here we use a monad transformer to encapsulate the novel recursion pattern in their generalized de Bruijn representation. It is named Scope to match up with the terminology from Conor McBride and James McKinna's "I am not a number: I am a free variable", while providing stronger type safety guarantees.

http://www.cs.st-andrews.ac.uk/~james/RESEARCH/notanum.pdf

There are three worked examples in the examples folder:

• Simple.hs provides an untyped lambda calculus with recursive let bindings.

• Derived.hs shows how much of the API can be automated with DeriveTraversable and adds combinators for building binders with pattern matching.

• Overkill.hs provides very strongly typed pattern matching many modern type extensions, including polymorphic kinds to ensure type safety. In general, the approach taken by Derived seems to deliver a better power to weight ratio.