Why can I not make String an instance of a typeclass?
Given :
data Foo =
FooString String
…
class Fooable a where --(is this a good way to name this?)
toFoo :: a -> Foo
I want to make String an instance of Fooable :
instance Fooable String where
toFoo = FooString
GHC then complains:
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'
If instead I use [Char] :
instance Fooable [Char] where
toFoo = FooString
GHC complains:
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]'
Question :
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
...
You're running into two limitations of classic Haskell98 typeclasses:
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 ab .. 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::
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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