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)
