Bound.TH

This is a Template Haskell module for deriving Applicative and Monad instances for data types.

Documentation

makeBound :: Name -> DecsQ

Use to automatically derive Applicative and Monad instances for your datatype.

In GHC 7.10 or later the DeriveAnyClass extension may be used to derive the Show1 and Read1 instances

{--}
{--}
{--}

import Bound          (Scope, makeBound)
import Prelude.Extras (Read1, Show1)

data Exp a 
  = V a 
  | App (Exp a) (Exp a) 
  | Lam (Scope () Exp a) 
  | I Int 
  deriving (Functor, Read, Read1, Show, Show1)

makeBound ''Exp

and in GHCi

ghci> :set -XDeriveAnyClass
ghci> :set -XDeriveFunctor 
ghci> :set -XTemplateHaskell 
ghci> import Bound          (Scope, makeBound)
ghci> import Prelude.Extras (Read1, Show1)
ghci> data Exp a = V a | App (Exp a) (Exp a) | Lam (Scope () Exp a) | I Int deriving (Functor, Read, Read1, Show, Show1); makeBound ''Exp

or

ghci> :{
ghci| data Exp a = V a | App (Exp a) (Exp a) | Lam (Scope () Exp a) | I Int deriving (Functor, Read, Read1, Show, Show1)
ghci| makeBound ''Exp
ghci| :}

If DeriveAnyClass is not used the instances must be declared explicitly:

data Exp a = V a | App (Exp a) (Exp a) | Lam (Scope () Exp a) | I Int deriving (Functor, Read, Show) instance Read1 Exp instance Show1 Exp

makeBound ''Exp @

or in GHCi:

ghci> :{
ghci| data Exp a = V a | App (Exp a) (Exp a) | Lam (Scope () Exp a) | I Int deriving (Functor, Read, Show)
ghci| instance Read1 Exp
ghci| instance Show1 Exp
ghci| makeBound ''Exp
ghci| :}

Eq and Ord instances need to be derived differently if the data type's immediate components include Scope (or other instances of Bound)

In a file with {--} at the top:

instance Eq1 Exp
deriving instance Eq a => Eq (Exp a)

instance Ord1 Exp
deriving instance Ord a => Ord (Exp a)

or in GHCi:

ghci> :set -XStandaloneDeriving 
ghci> deriving instance Eq a => Eq (Exp a); instance Eq1 Exp
ghci> deriving instance Ord a => Ord (Exp a); instance Ord1 Exp

because their Eq and Ord instances require Exp to be a Monad:

  instance (Monad f, Eq b, Eq1 f, Eq a)    => Eq (Scope b f a)
  instance (Monad f, Ord b, Ord1 f, Ord a) => Ord (Scope b f a)

Does not work yet for components that are lists or instances of Functor or with a great deal other things.