为什么我不能使 String 成为类型类的实例？

鉴于 :

data Foo =
FooString String
…


class Fooable a where --(is this a good way to name this?)
toFoo :: a -> Foo

我想使 String成为 Fooable的一个实例:

instance Fooable String where
toFoo = FooString

GHC 随后抱怨道:

Illegal instance declaration for `Fooable String'
(All instance types must be of the form (T t1 ... tn)
where T is not a synonym.
Use -XTypeSynonymInstances if you want to disable this.)
In the instance declaration for `Fooable String'

如果我使用 [Char]:

instance Fooable [Char] where
toFoo = FooString

GHC 抱怨道:

Illegal instance declaration for `Fooable [Char]'
(All instance types must be of the form (T a1 ... an)
where a1 ... an are type *variables*,
and each type variable appears at most once in the instance head.
Use -XFlexibleInstances if you want to disable this.)
In the instance declaration for `Fooable [Char]'

问题 :

  • 为什么我不能创建类型类的 String 和 instance？
  • 如果我多加一面国旗 GHC 似乎愿意放过我。这是个好主意吗？
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小开

You're running into two limitations of classic Haskell98 typeclasses:

  • they disallow type synonyms in instances
  • they disallow nested types that don't in turn contain type variables.

These onerous restrictions are lifted by two language extensions:

  • -XTypeSynonymInstances

which allows you to use type synoyms (like String for [Char]), and:

  • -XFlexibleInstances

which lift the restrictions on instance types being of the form T a b .. where the parameters are type variables. The -XFlexibleInstances flag allows the head of the instance declaration to mention arbitrary nested types.

Note that lifting these restrictions can sometimes lead to overlapping instances, at which point, an additional language extension might be needed to resolve the ambiguity, allowing GHC to pick an instance for you.

References::

小开
最佳答案

This is because String is just a type alias for [Char], which is just the application of the type constructor [] on the type Char, so this would be of the form ([] Char). which is not of the form (T a1 .. an) because Char is not a type variable.

The reason for this restriction is to prevent overlapping instances. For example, let's say you had an instance Fooable [Char], and then someone later came along and defined an instance Fooable [a]. Now the compiler won't be able to figure out which one you wanted to use, and will give you an error.

By using -XFlexibleInstances, you're basically promising to the compiler that you won't define any such instances.

Depending on what you're trying to accomplish, it might be better to define a wrapper:

newtype Wrapper = Wrapper String
instance Fooable Wrapper where
...
小开

Adding to these answers, if you are not comfortable with lifting the restrictions, there may be cases where it could make sense to wrap your String in a newtype, which can be an instance of a class. The tradeoff would be potential ugliness, having to wrap and unwrap in your code.

小开

FlexibleInstances are not a good answer in most cases. Better alternatives are wrapping the String in a newtype or introduce a helper class like so:

class Element a where
listToFoo :: [a] -> Foo


instance Element Char where
listToFoo = FooString


instance Element a => Fooable [a] where
toFoo = listToFoo

See also: http://www.haskell.org/haskellwiki/List_instance


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