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