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.

Also works for components that are lists or instances of Functor, but still does not work for a great deal of other things.

deriving-compat package may be used to derive the Show1 and Read1 instances

{-# LANGUAGE DeriveFunctor      #-}
{-# LANGUAGE TemplateHaskell    #-}

import Bound                (Scope, makeBound)
import Data.Functor.Classes (Show1, Read1, shosPrec1, readsPrec1)
import Data.Deriving        (deriveShow1, deriveRead1)

data Exp a
  = V a
  | App (Exp a) (Exp a)
  | Lam (Scope () Exp a)
  | ND [Exp a]
  | I Int
  deriving (Functor)

makeBound ''Exp
deriveShow1 ''Exp
deriveRead1 ''Exp
instance Read a => Read (Exp a) where readsPrec = readsPrec1
instance Show a => Show (Exp a) where showsPrec = showsPrec1

and in GHCi

ghci> :set -XDeriveFunctor
ghci> :set -XTemplateHaskell
ghci> import Bound                (Scope, makeBound)
ghci> import Data.Functor.Classes (Show1, Read1, showsPrec1, readsPrec1)
ghci> import Data.Deriving        (deriveShow1, deriveRead1)
ghci> :{
ghci| data Exp a = V a | App (Exp a) (Exp a) | Lam (Scope () Exp a) | ND [Exp a] | I Int deriving (Functor)
ghci| makeBound ''Exp
ghci| deriveShow1 ''Exp
ghci| deriveRead1 ''Exp
ghci| instance Read a => Read (Exp a) where readsPrec = readsPrec1
ghci| instance Show a => Show (Exp a) where showsPrec = showsPrec1
ghci| :}

Eq and Ord instances can be derived similarly

import Data.Functor.Classes (Eq1, Ord1, eq1, compare1)
import Data.Deriving        (deriveEq1, deriveOrd1)

deriveEq1 ''Exp
deriveOrd1 ''Exp
instance Eq a => Eq (Exp a) where (==) = eq1
instance Ord a => Ord (Exp a) where compare = compare1

or in GHCi:

ghci> import Data.Functor.Classes (Eq1, Ord1, eq1, compare1)
ghci> import Data.Deriving        (deriveEq1, deriveOrd1)
ghci> :{
ghci| deriveEq1 ''Exp
ghci| deriveOrd1 ''Exp
ghci| instance Eq a => Eq (Exp a) where (==) = eq1
ghci| instance Ord a => Ord (Exp a) where compare = compare1
ghci| :}

We cannot automatically derive Eq and Ord using the standard GHC mechanism, because 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)