Add bitSizeMaybe to Bits, and add FiniteBits class

1 files changed, 23 insertions, 2 deletions

diff --git a/libraries/base/Data/Bits.hs b/libraries/base/Data/Bits.hs
index 6d97352382..97edb3485a 100644
--- a/libraries/base/Data/Bits.hs
+++ b/libraries/base/Data/Bits.hs
@@ -30,6 +30,7 @@ module Data.Bits (
     clearBit,          -- :: a -> Int -> a
     complementBit,     -- :: a -> Int -> a
     testBit,           -- :: a -> Int -> Bool
+    bitSizeMaybe,
     bitSize,           -- :: a -> Int
     isSigned,          -- :: a -> Bool
     shiftL, shiftR,    -- :: a -> Int -> a
@@ -37,6 +38,7 @@ module Data.Bits (
     rotateL, rotateR,  -- :: a -> Int -> a
     popCount           -- :: a -> Int
   ),
+  FiniteBits(finiteBitSize),
 
   bitDefault,
   testBitDefault,
@@ -54,6 +56,7 @@ module Data.Bits (
 #endif
 
 #ifdef __GLASGOW_HASKELL__
+import Data.Maybe
 import GHC.Enum
 import GHC.Num
 import GHC.Base
@@ -152,6 +155,12 @@ class Eq a => Bits a where
     testBit           :: a -> Int -> Bool
 
     {-| Return the number of bits in the type of the argument.  The actual
+        value of the argument is ignored.  Returns Nothing
+        for types that do not have a fixed bitsize, like 'Integer'.
+        -}
+    bitSizeMaybe      :: a -> Maybe Int
+
+    {-| Return the number of bits in the type of the argument.  The actual
         value of the argument is ignored.  The function 'bitSize' is
         undefined for types that do not have a fixed bitsize, like 'Integer'.
         -}
@@ -238,6 +247,9 @@ class Eq a => Bits a where
         known as the population count or the Hamming weight. -}
     popCount          :: a -> Int
 
+class Bits b => FiniteBits b where
+    finiteBitSize :: b -> Int
+
 -- | Default implementation for 'bit'.
 --
 -- Note that: @bitDefault i = 1 `shiftL` i@
@@ -297,7 +309,8 @@ instance Bits Int where
         !x'# = int2Word# x#
         !i'# = word2Int# (int2Word# i# `and#` int2Word# (wsib -# 1#))
         !wsib = WORD_SIZE_IN_BITS#   {- work around preprocessor problem (??) -}
-    bitSize  _             = WORD_SIZE_IN_BITS
+    bitSizeMaybe i         = Just (finiteBitSize i)
+    bitSize i              = finiteBitSize i
 
     popCount (I# x#) = I# (word2Int# (popCnt# (int2Word# x#)))
 
@@ -338,6 +351,9 @@ instance Bits Int where
 
     isSigned _             = True
 
+instance FiniteBits Int where
+    finiteBitSize _ = WORD_SIZE_IN_BITS
+
 #ifdef __NHC__
 foreign import ccall nhc_primIntAnd :: Int -> Int -> Int
 foreign import ccall nhc_primIntOr  :: Int -> Int -> Int
@@ -371,11 +387,15 @@ instance Bits Word where
         where
         !i'# = word2Int# (int2Word# i# `and#` int2Word# (wsib -# 1#))
         !wsib = WORD_SIZE_IN_BITS#  {- work around preprocessor problem (??) -}
-    bitSize  _               = WORD_SIZE_IN_BITS
+    bitSizeMaybe i           = Just (finiteBitSize i)
+    bitSize i                = finiteBitSize i
     isSigned _               = False
     popCount (W# x#)         = I# (word2Int# (popCnt# x#))
     bit                      = bitDefault
     testBit                  = testBitDefault
+
+instance FiniteBits Word where
+    finiteBitSize _ = WORD_SIZE_IN_BITS
 #endif
 
 instance Bits Integer where
@@ -413,6 +433,7 @@ instance Bits Integer where
 
    rotate x i = shift x i   -- since an Integer never wraps around
 
+   bitSizeMaybe _ = Nothing
    bitSize _  = error "Data.Bits.bitSize(Integer)"
    isSigned _ = True