修正第 6 章的翻译

[huangzworks]

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[scarletsky]

考虑下加入 FlexibleInstances 和 TypeSynonymInstances 的区别？

Making an instance with a type synonym 一节第二段 ，里面有位 piu 的用户的留言写得挺好的，我贴一下：

piu 2010-11-01

instance JSON [Char] needs the FlexibleInstances pragma,
instance JSON String needs the TypeSynonymInstances pragma.

these are different problems, I think, even if they might often be related.

simple example:
----------------------
given the following type and typeclass:

data (Num a) => Vector a = Vector a a
deriving (Show)

class MyClass a where
myFun :: a -> a

it seems perfectly ok to define (without the need of any pragma)

instance MyClass (Vector a) where
myFun = id

myFun (Vector 1 2) :: Vector Int gives Vector 1 2, myFun (Vector 1 2) :: Vector Double gives Vector 1.0 2.0 instead.

but if you try to define

instance MyClass (Vector Int) where etc…
instance MyClass (Vector Double) where etc…

- perhaps because you want to handle those types of vectors differently - you will need the FlexibleInstances pragma. and if you try to work around this problem by introducing two new type synonyms

type VectorInt = Vector Int
type VectorDouble = Vector Double

and using them in the instance declaration

instance MyClass VectorInt where etc…

you will need the TypeSynonymInstances pragma instead.