[Haskell-cafe] Overlapping instances with "instance F a => G a"

[Bryan Gardiner]

Hi cafe!  Happy New Year.

I'm writing a version of the Binary typeclass that encodes values with
host endianness, and I have the code at the bottom of the message.
HostBinary provides the encoding/decoding interface, and with
HostBinaryNum I want to be able to write only e.g. "instance
HostBinaryNum Int32" for numeric types.  I had the manual HostBinary
Word8 and HostBinary Int8 instances before I wrote HostBinaryNum.
What I can't see is why I get this error at all:

  Binary.hs:21:29:
      Overlapping instances for HostBinary Word8
        arising from a use of ‘hget’
      Matching instances:
        instance HostBinary Word8 -- Defined at Binary.hs:16:10
        instance HostBinaryNum a => HostBinary a
          -- Defined at Binary.hs:26:10
      In the second argument of ‘fmap’, namely ‘(hget :: Get Word8)’
      In the expression: fmap fromIntegral (hget :: Get Word8)
      In an equation for ‘hget’:
          hget = fmap fromIntegral (hget :: Get Word8)

and also why commenting out either the HostBinary Int8 or the
HostBinaryNum a => HostBinary a instances fixes the problem; and
yet, the HostBinaryNum Word8 is accepted!

Doesn't "HostBinaryNum a => HostBinary a" create a HostBinary instance
for all instances of HostBinaryNum only?  So why would it cause
problems with an Int8 instance, and why isn't the HostBinaryNum Word8
instance needed to trigger a collision with the explicit HostBinary
Word8?  GHC 7.8.3, if it matters.

Thanks for any clarifiation you can provide,
Bryan

Binary.hs:

  {-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
  
  module Binary where
  
  import Data.Binary.Get
  import Data.Binary.Put
  import Data.Bits
  import Data.Int
  import Data.Word
  import Foreign.Storable
  
  class HostBinary a where
    hget :: Get a
    hput :: a -> Put
  
  instance HostBinary Word8 where
    hget = getWord8
    hput = putWord8
  
  instance HostBinary Int8 where
    hget = fmap fromIntegral (hget :: Get Word8)
    hput = hput . (fromIntegral :: Int8 -> Word8)
  
  class (Bits a, Integral a, Storable a) => HostBinaryNum a
  
  instance HostBinaryNum a => HostBinary a where
    hget = getNum
    hput = putNum
  
  --instance HostBinaryNum Word8
  
  getNum :: (Bits a, Integral a, Storable a) => Get a
  getNum = undefined
  
  putNum :: (Bits a, Integral a, Storable a) => a -> Put
  putNum = undefined
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