Adding Read instances for HList

[David Feuer]

One option looks something like the code below. Another option would
mirror the current Show instances by pushing "H[" back onto the state
each time an item is parsed (I don't think that's a particularly clean
approach, but it's an option).


{-# LANGUAGE
  DataKinds
 ,KindSignatures
 ,FlexibleContexts
 ,FlexibleInstances
 ,TypeOperators
 ,GADTs
 ,ScopedTypeVariables
 #-}

module MyHList where
import GHC.Read
import Text.ParserCombinators.ReadPrec
import qualified Text.Read.Lex as L

-- This declaration is copied from `Data.HList.HList`.
data HList (l::[*]) where
  HNil  :: HList '[]
  HCons :: e -> HList l -> HList (e ': l)

infixr 2 `HCons`


instance Read (HList '[]) where
  readPrec = parens
    ( do
        expectP (L.Ident "H")
        expectP (L.Punc "[")
        expectP (L.Punc "]")
        return HNil
    )

instance forall e . Read e => Read (HList (e ': '[])) where
  readPrec = parens
    ( do
        expectP (L.Ident "H")
        expectP (L.Punc "[")
        e <- reset $ (readPrec::ReadPrec e)
        expectP (L.Punc "]")
        return (e `HCons` HNil)
    )

instance forall e f (l::[*]) . (Read e, ReadTl (HList (f ': l))) =>
   Read (HList (e ': f ': l)) where
  readPrec = parens
    ( do
        expectP (L.Ident "H")
        expectP (L.Punc "[")
        e <- reset $ (readPrec::ReadPrec e)
        expectP (L.Punc ",")
        rest <- readTl :: ReadPrec (HList (f ': l))
        return (e `HCons` rest)
    )

class ReadTl l where
  readTl :: ReadPrec l

instance forall e . Read e => ReadTl (HList (e ': '[])) where
  {-# INLINE readTl #-}
  readTl = do
    e <- reset $ (readPrec::ReadPrec e)
    expectP (L.Punc "]")
    return $ e `HCons` HNil

instance forall e f (l::[*]) . (Read e, ReadTl (HList (f ': l)))
             => ReadTl (HList (e ': f ': l)) where
  {-# INLINE readTl #-}
  readTl = do
    e <- reset $ (readPrec::ReadPrec e)
    expectP (L.Punc ",")
    rest <- readTl
    return (e `HCons` rest)