Bound

Goals

This library provides convenient combinators for working with "locally-nameless" terms. These can be useful when writing a type checker, evalator, parser, or pretty printer for terms that contain binders like forall or lambda, as they ease the task of avoiding variable capture and testing for alpha-equivalence.

See the documentation on hackage for more information, but here is an example:

 import Bound
 import Prelude.Extras

 infixl 9 :@
 data Exp a = V a | Exp a :@ Exp a | Lam (Scope () Exp a)
   deriving (Eq,Ord,Show,Read,Functor,Foldable,Traversable)

 instance Eq1 Exp   where (==#)      = (==)
 instance Ord1 Exp  where compare1   = compare
 instance Show1 Exp where showsPrec1 = showsPrec
 instance Read1 Exp where readsPrec1 = readsPrec
 instance Applicative Exp where pure = V; (<*>) = ap

 instance Monad Exp where
   return = V
   V a      >>= f = f a
   (x :@ y) >>= f = (x >>= f) :@ (y >>= f)
   Lam e    >>= f = Lam (e >>>= f)

 lam :: Eq a => a -> Exp a -> Exp a
 lam v b = Lam (abstract1 v b)

 whnf :: Exp a -> Exp a
 whnf (f :@ a) = case whnf f of
   Lam b -> whnf (instantiate1 a b)
   f'    -> f' :@ a
 whnf e = e

There are longer examples in the examples/ folder.

Contact Information

Contributions and bug reports are welcome!

Please feel free to contact me through github or on the #haskell IRC channel on irc.freenode.net.

-Edward Kmett