BSD3 licensed by Edward A. Kmett
Maintained by Edward A. Kmett

Module documentation for 1.0.6

Bound

Hackage Build Status

Goals

This library provides convenient combinators for working with “locally-nameless” terms. These can be useful when writing a type checker, evaluator, 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:

{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE DeriveFoldable #-}
{-# LANGUAGE DeriveTraversable #-}
{-# LANGUAGE TemplateHaskell #-}

import Bound
import Control.Applicative
import Control.Monad
import Data.Functor.Classes
import Data.Foldable
import Data.Traversable
import Data.Eq.Deriving (deriveEq1)      -- these two are from the
import Text.Show.Deriving (deriveShow1)  -- deriving-compat package

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

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

deriveEq1 ''Exp
deriveShow1 ''Exp

main :: IO ()
main = do
  let term = lam 'x' (V 'x') :@ V 'y'
  print term         -- Lam (Scope (V (B ()))) :@ V 'y'
  print $ whnf term  -- V 'y'

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

Changes

2.0.1

  • Add abstractEither and instantiateEither to Bound.Scope, and add abstractEitherName and instantiateEitherName to Bound.Scope.Name
  • Add Generic(1) instances for Name and Scope
  • Support doctest-0.12

2

  • GHC 8.0 and 8.2 support
  • Converted from prelude-extras to transformers + transformers-compat for the Eq1, Ord1, Show1, and Read1 functionality.
  • makeBound supports Functor components
  • Add MFunctor instance for Scope
  • Add NFData instances for Name, Scope, and Var
  • Revamp Setup.hs to use cabal-doctest. This makes it build with Cabal-1.25, and makes the doctests work with cabal new-build and sandboxes.

1.0.7

  • Added an -f-template-haskell option to allow disabling template-haskell support. This is an unsupported configuration but may be useful for expert users in sandbox configurations.
  • Support cereal 0.5

1.0.6

  • Compiles warning-free on GHC 7.10

1.0.5

  • Widened version bound on bifunctors.
  • Widened version bound on profunctors.

1.0.4

  • Widened version bound on transformers.

1.0.3

  • Added bitransverseScope.

1.0.2

  • Removed unneccesary constraint on hoistScope.

1.0.1

  • Added a monomorphic hoistScope for Bound.Scope.Simple

1.0

  • Added instances for Bound for all of the mtl monads.
  • Added Data and Typeable support to both versions of Scope
  • Added the missing Applictive instance to Bound.Scope.Simple
  • Moved hoistScope, bitraverseScope, transverseScope, and instantiateVars here from the ermine compiler.

0.9.1.1

  • Updated to work with bifunctors 4.0

0.9.1

  • Updated to work with comonad 4.0 and profunctors 4.0

0.9

  • Added the missing instance for Applicative (Scope b f)

0.8.1

  • SafeHaskell support

0.8

  • Added Serial, Binary and Serialize instances for Scope.

0.7

  • Added Hashable, Hashable1 and Hashable2 instances where appropriate for Name, Var and Scope.

0.6.1

  • More aggressive inlining
  • Added unvar, _B, _F to Bound.Var.
  • Added _Name to Bound.Name.

0.6

  • Support for prelude-extras 0.3

0.5.1

  • Removed my personal inter-package dependency upper bounds
  • Updated doctest suite to use exact versions.

0.5

  • Created a doctest-based test suite
  • Added many examples
  • 100% haddock coverage
  • Added the Name Comonad, to help retain names for bound variables.
  • Bumped dependencies
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