當您想將自定義型別轉換為位元組串時，您將執行以下操作：
data Foo = Foo {value1 :: Int}
instance Binary Foo where
get =liftM Foo get
put b = put (value1 b)
如果您有一個具有多個可行值的型別，例如：
data Foo2 = Foo2A Int | Foo2B Int
instance Binary Foo2 where
get = do flag <- getWord8
case flag of
0 -> fmap Foo2A get
1 -> fmap Foo2B get
put (Foo2A i) = do put (0 :: Word8)
put i
put (Foo2B i) = do put (1 :: Word8)
put i
但是，如果您有這樣的型別（以下...），我該怎么做？：
data Foo3 = Foo3A Int | Foo3B
instance Binary Foo3 where
get = do flag <- getWord8
case flag of
0 -> fmap Foo3A get
1 -> ....????? Foo3B has no value - only Data Constructor
put (Foo3A i) = do put (0 :: Word8)
put i
put (Foo3B) = put (1 :: Word8)
uj5u.com熱心網友回復：
您還可以派生這些實體：
newtype Foo = Foo {value1 :: Int}
deriving newtype Binary
data Foo2 = Foo2A Int | Foo2B Int
deriving stock Generic
deriving anyclass Binary
data Foo3 = Foo3A Int | Foo3B
deriving stock Generic
deriving anyclass Binary
uj5u.com熱心網友回復：
為了匹配您所寫的內容put，您希望pure Foo3B在get.
轉載請註明出處，本文鏈接：https://www.uj5u.com/houduan/434058.html
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