Implement precedence-handling bijective read/show

refactors and reimplements Network.Socket.ReadShow to allow
for precedence-sensitive Read/Show instances built on an underlying
bijective framework. Behavior conforms to derived read/show instances
by default but short-circuits to fixed strings when input matches
element of a list of paired elements defining a partial bijection.

adds several flexible helper-functions to Network.Socket.ReadShow
to allow for minimal-boilerplate implementations of bijective
read/show for arbitrary types in future.

Adds more descriptive documentation for non-obvious properties
of Network.Socket.ReadShow types and functions

reimplements instance declarations of read/show for types already
using bijective read/show definitions

Adds cases to Network.SocketSpec test suite to ensure that ReadShow-based Show instances produce expected output for each pattern-bijection type

Adds quickcheck-dependent (updated cabal file) tests to ensure
roundtrip equality for `read.show` for all types using ReadShow-based
instances over arbitrary values, specifically biased towards pattern
synonym values

Network/Socket/Options.hsc

@@ -390,8 +390,8 @@ getSockOpt s (SockOpt level opt) = do
         peek ptr
 
 
-socketOptionPairs :: [Pair SocketOption String]
-socketOptionPairs =
+socketOptionBijection :: Bijection SocketOption String
+socketOptionBijection =
     [ (UnsupportedSocketOption, "UnsupportedSocketOption")
     , (Debug, "Debug")
     , (ReuseAddr, "ReuseAddr")
@@ -426,21 +426,19 @@ socketOptionPairs =
     , (RecvIPv6PktInfo, "RecvIPv6PktInfo")
     ]
 
-socketOptionBijection :: Bijection SocketOption String
-socketOptionBijection = Bijection{..}
-    where
-        cso = "CustomSockOpt"
-        unCSO = \(CustomSockOpt nm) -> nm
-        defFwd = defShow cso unCSO _show
-        defBwd = defRead cso CustomSockOpt _parse
-        pairs = socketOptionPairs
-
 instance Show SocketOption where
-    show = forward socketOptionBijection
+    showsPrec = bijectiveShow socketOptionBijection def
+      where
+        defname = "SockOpt"
+        unwrap = \(CustomSockOpt nm) -> nm
+        def = defShow defname unwrap showIntInt
 
-instance Read SocketOption where
-    readPrec = tokenize $ backward socketOptionBijection
 
+instance Read SocketOption where
+    readPrec = bijectiveRead socketOptionBijection def
+      where
+        defname = "SockOpt"
+        def = defRead defname CustomSockOpt readIntInt
 
 foreign import CALLCONV unsafe "getsockopt"
   c_getsockopt :: CInt -> CInt -> CInt -> Ptr a -> Ptr CInt -> IO CInt

Network/Socket/Posix/Cmsg.hsc

@@ -229,8 +229,8 @@ instance Storable IPv6PktInfo where
 instance ControlMessage Fd where
     controlMessageId = CmsgIdFd
 
-cmsgIdPairs :: [Pair CmsgId String]
-cmsgIdPairs =
+cmsgIdBijection :: Bijection CmsgId String
+cmsgIdBijection =
     [ (UnsupportedCmsgId, "UnsupportedCmsgId")
     , (CmsgIdIPv4TTL, "CmsgIdIPv4TTL")
     , (CmsgIdIPv6HopLimit, "CmsgIdIPv6HopLimit")
@@ -241,17 +241,17 @@ cmsgIdPairs =
     , (CmsgIdFd, "CmsgIdFd")
     ]
 
-cmsgIdBijection :: Bijection CmsgId String
-cmsgIdBijection = Bijection{..}
-    where
+instance Show CmsgId where
+    showsPrec = bijectiveShow cmsgIdBijection def
+      where
         defname = "CmsgId"
         unId = \(CmsgId l t) -> (l,t)
-        defFwd = defShow defname unId _show
-        defBwd = defRead defname (uncurry CmsgId) _parse
-        pairs = cmsgIdPairs
-
-instance Show CmsgId where
-    show = forward cmsgIdBijection
+        def = defShow defname unId showIntInt
 
 instance Read CmsgId where
-    readPrec = tokenize $ backward cmsgIdBijection
+    readPrec = bijectiveRead cmsgIdBijection def
+      where
+        defname = "CmsgId"
+        def = defRead defname (uncurry CmsgId) readIntInt
+
+

Network/Socket/ReadShow.hs

@@ -1,10 +1,16 @@
 {-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE PatternGuards #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE RankNTypes #-}
 
 
 module Network.Socket.ReadShow where
 
+import Text.Read ((<++))
 import qualified Text.Read as P
+import qualified Text.Read.Lex as P
+import Control.Monad (mzero)
 
 -- type alias for individual correspondences of a (possibly partial) bijection
 type Pair a b = (a, b)
@@ -19,77 +25,112 @@ eqFst x = \(x',_) -> x' == x
 eqSnd :: Eq b => b -> (a, b) -> Bool
 eqSnd y = \(_,y') -> y' == y
 
--- | Return RHS element that is paired with provided LHS,
---   or apply a default fallback function if the list is partial
-lookForward :: Eq a => (a -> b) -> [Pair a b] -> a -> b
-lookForward defFwd ps x
-  = case filter (eqFst x) ps of
-      (_,y):_ -> y
-      [] -> defFwd x
-
--- | Return LHS element that is paired with provided RHS,
---   or apply a default fallback function if the list is partial
-lookBackward :: Eq b => (b -> a) -> [Pair a b] -> b -> a
-lookBackward defBwd ps y
-  = case filter (eqSnd y) ps of
-      (x,_):_ -> x
-      [] -> defBwd y
-
-data Bijection a b
-   = Bijection
-     { defFwd :: a -> b
-     , defBwd :: b -> a
-     , pairs  :: [Pair a b]
-     }
-
--- | apply a bijection over an LHS-value
-forward :: (Eq a) => Bijection a b -> a -> b
-forward Bijection{..} = lookForward defFwd pairs
-
--- | apply a bijection over an RHS-value
-backward :: (Eq b) => Bijection a b -> b -> a
-backward Bijection{..} = lookBackward defBwd pairs
-
--- | show function for Int-like types that encodes negative numbers
--- with leading '_' instead of '-'
-_showInt :: (Show a, Num a, Ord a) => a -> String
-_showInt n | n < 0 = let ('-':s) = show n in '_':s
-           | otherwise = show n
-
--- | parse function for Int-like types that interprets leading '_'
---   as if it were '-' instead
-_readInt :: (Read a) => String -> a
-_readInt ('_':s) = read $ '-':s
-_readInt s = read s
-
-
--- | parse a quote-separated pair into a tuple of Int-like values
---   should not be used if either type might have
---   literal quote-characters in the Read pre-image
-_parse :: (Read a, Read b) => String -> (a, b)
-_parse xy =
-  let (xs, '\'':ys) = break (=='\'') xy
-   in (_readInt xs, _readInt ys)
-{-# INLINE _parse #-}
-
--- | inverse function to _parse
---   show a tuple of Int-like values as quote-separated strings
-_show :: (Show a, Num a, Ord a,  Show b, Num b, Ord b) => (a, b) -> String
-_show (x, y) = _showInt x ++ "'" ++ _showInt y
-{-# INLINE _show #-}
-
-defShow :: Eq a => String -> (a -> b) -> (b -> String) -> (a -> String)
-defShow name unwrap sho = \x -> name ++ (sho . unwrap $ x)
+
+-- | Unified automorphic involution over @Either a b@ that converts between
+--   LHS and RHS elements of a list of @Pair a b@ mappings and is the identity
+--   function if no matching pair is found
+--
+--   If list contains duplicate matches, short-circuits to the first matching @Pair@
+lookBetween :: (Eq a, Eq b) => [Pair a b] -> Either a b -> Either a b
+lookBetween ps = \case
+  Left  x | (_,y):_ <- filter (eqFst x) ps -> Right y
+  Right y | (x,_):_ <- filter (eqSnd y) ps -> Left  x
+  z -> z
+
+-- Type alias for partial bijections between two types, consisting of a list
+-- of individual correspondences that are checked in order and short-circuit
+-- on first match
+--
+-- Depending on how this is used, may not actually be a true bijection over
+-- the partial types, as no overlap-checking is currently implemented. If
+-- overlaps are unavoidable, the canonical short-circuit pair should appear
+-- first to avoid round-trip inconsistencies.
+type Bijection a b = [Pair a b]
+
+-- | Helper function for prefixing an optional constructor name before arbitrary values,
+-- which only enforces high precedence on subsequent output if the constructor name is not
+-- blank and space-separates for non-blank constructor names
+namePrefix :: Int -> String -> (Int -> b -> ShowS) -> b -> ShowS
+namePrefix i name f x
+    | null name = f i x
+    | otherwise = showParen (i > app_prec) $ showString name . showChar ' ' . f (app_prec+1) x
+{-# INLINE namePrefix #-}
+
+-- | Helper function for defining bijective Show instances that represents
+-- a common use-case where a constructor (or constructor-like pattern) name
+-- (optionally) precedes an internal value with a separate show function
+defShow :: Eq a => String -> (a -> b) -> (Int -> b -> ShowS) -> (Int -> a -> ShowS)
+defShow name unwrap shoPrec = \i x -> namePrefix i name shoPrec (unwrap x)
 {-# INLINE defShow #-}
 
-defRead :: Read a => String -> (b -> a) -> (String -> b) -> (String -> a)
-defRead name wrap red = \s ->
-  case splitAt (length name) s of
-    (x, sn) | x == name -> wrap $ red sn
-    _ -> error $ "defRead: unable to parse " ++ show s
+-- Helper function for stripping an optional constructor-name prefix before parsing
+-- an arbitrary value, which only consumes an extra token and increases precedence
+-- if the provided name prefix is non-blank
+expectPrefix :: String -> P.ReadPrec a -> P.ReadPrec a
+expectPrefix name pars
+    | null name = pars
+    | otherwise = do
+        P.lift $ P.expect $ P.Ident name
+        P.step pars
+{-# INLINE expectPrefix #-}
+
+-- | Helper function for defining bijective Read instances that represent a
+-- common use case where a constructor (or constructor-like pattern) name
+-- (optionally) precedes an internal value with a separate parse function
+defRead :: Eq a => String -> (b -> a) -> P.ReadPrec b -> P.ReadPrec a
+defRead name wrap redPrec = expectPrefix name $ wrap <$> redPrec
 {-# INLINE defRead #-}
 
--- | Apply a precedence-invariant one-token parse function within ReadPrec monad
-tokenize :: (String -> a) -> P.ReadPrec a
-tokenize f = P.lexP >>= \(P.Ident x) -> return $ f x
-{-# INLINE tokenize #-}
+-- | Alias for showsPrec that pairs well with `_readInt`
+_showInt :: (Show a) => Int -> a -> ShowS
+_showInt = showsPrec
+{-# INLINE _showInt #-}
+
+-- | More descriptive alias for `safeInt`
+_readInt :: (Bounded a, Integral a) => P.ReadPrec a
+_readInt = safeInt
+{-# INLINE _readInt #-}
+
+-- | show two elements of a tuple separated by a space character
+-- inverse function to readIntInt when used on integer-like values
+showIntInt :: (Show a, Show b) => Int -> (a, b) -> ShowS
+showIntInt i (x, y) = _showInt i x . showChar ' ' . _showInt i y
+{-# INLINE showIntInt #-}
+
+-- | consume and return two integer-like values from two consecutive lexical tokens
+readIntInt :: (Bounded a, Integral a, Bounded b, Integral b) => P.ReadPrec (a, b)
+readIntInt = do
+  x <- _readInt
+  y <- _readInt
+  return (x, y)
+{-# INLINE readIntInt #-}
+
+bijectiveShow :: (Eq a) => Bijection a String -> (Int -> a -> ShowS) -> (Int -> a -> ShowS)
+bijectiveShow bi def = \i x ->
+    case lookBetween bi (Left x) of
+        Right y -> showString y
+        _ -> def i x
+
+bijectiveRead :: (Eq a) => Bijection a String -> P.ReadPrec a -> P.ReadPrec a
+bijectiveRead bi def = P.parens $ bijective <++ def
+  where
+    bijective = do
+      (P.Ident y) <- P.lexP
+      case lookBetween bi (Right y) of
+          Left x -> return x
+          _ -> mzero
+
+app_prec :: Int
+app_prec = 10
+{-# INLINE app_prec #-}
+
+-- Parse integral values with type-specific overflow and underflow bounds-checks
+safeInt :: forall a. (Bounded a, Integral a) => P.ReadPrec a
+safeInt = do
+    i <- signed
+    if (i >= fromIntegral (minBound :: a) && i <= fromIntegral (maxBound :: a))
+    then return $ fromIntegral i
+    else mzero
+  where
+    signed :: P.ReadPrec Integer
+    signed = P.readPrec

Network/Socket/Types.hsc

@@ -91,9 +91,7 @@ import GHC.IORef (IORef (..))
 import GHC.STRef (STRef (..))
 import GHC.IO (IO (..))
 
-import Text.Read ((<++))
 import qualified Text.Read as P
-import qualified Text.Read.Lex as P
 
 #if defined(DOMAIN_SOCKET_SUPPORT)
 import Foreign.Marshal.Array
@@ -1316,8 +1314,8 @@ instance Storable In6Addr where
 ------------------------------------------------------------------------
 -- Read and Show instance for pattern-based integral newtypes
 
-socktypePairs :: [Pair SocketType String]
-socktypePairs =
+socktypeBijection :: Bijection SocketType String
+socktypeBijection =
     [ (UnsupportedSocketType, "UnsupportedSocketType")
     , (Stream, "Stream")
     , (Datagram, "Datagram") 
@@ -1327,22 +1325,20 @@ socktypePairs =
     , (NoSocketType, "NoSocketType")
     ]
 
-socktypeBijection :: Bijection SocketType String
-socktypeBijection = Bijection{..}
-    where
-      gst = "GeneralSocketType"
-      defFwd = defShow gst packSocketType _showInt
-      defBwd = defRead gst unpackSocketType _readInt
-      pairs = socktypePairs
-
 instance Show SocketType where
-    show = forward socktypeBijection
+    showsPrec = bijectiveShow socktypeBijection def
+      where
+        gst = "GeneralSocketType"
+        def = defShow gst packSocketType _showInt
 
 instance Read SocketType where
-    readPrec = tokenize $ backward socktypeBijection
+    readPrec = bijectiveRead socktypeBijection def
+      where
+        gst = "GeneralSocketType"
+        def = defRead gst unpackSocketType _readInt
 
-familyPairs :: [Pair Family String]
-familyPairs =
+familyBijection :: Bijection Family String
+familyBijection =
     [ (UnsupportedFamily, "UnsupportedFamily")
     , (AF_UNSPEC, "AF_UNSPEC")
     , (AF_UNIX, "AF_UNIX")
@@ -1412,19 +1408,17 @@ familyPairs =
     , (AF_CAN, "AF_CAN")
     ]
 
-familyBijection :: Bijection Family String
-familyBijection = Bijection{..}
-    where
-      gf = "GeneralFamily"
-      defFwd = defShow gf packFamily _showInt
-      defBwd = defRead gf unpackFamily _readInt
-      pairs = familyPairs
-
 instance Show Family where
-    show = forward familyBijection
+    showsPrec = bijectiveShow familyBijection def
+      where
+        gf = "GeneralFamily"
+        def = defShow gf packFamily _showInt
 
 instance Read Family where
-    readPrec = tokenize $ backward familyBijection
+    readPrec = bijectiveRead familyBijection def
+      where
+        gf = "GeneralFamily"
+        def = defRead gf unpackFamily _readInt
 
 -- Print "n" instead of "PortNum n".
 instance Show PortNumber where
@@ -1434,22 +1428,6 @@ instance Show PortNumber where
 instance Read PortNumber where
   readPrec = safeInt
 
-app_prec :: Int
-app_prec = 10
-
--- Accept negative values only in parens and check for overflow.
-safeInt :: forall a. (Bounded a, Integral a) => P.ReadPrec a
-safeInt = do
-    i <- P.parens unsigned <++ P.parens (P.prec app_prec negative)
-    if (i >= fromIntegral (minBound :: a) && i <= fromIntegral (maxBound :: a))
-    then return $ fromIntegral i
-    else mzero
-  where
-    unsigned :: P.ReadPrec Integer
-    unsigned = P.lift P.readDecP
-    negative :: P.ReadPrec Integer
-    negative = P.readPrec
-
 ------------------------------------------------------------------------
 -- Helper functions