[Haskell-cafe] instance of String illegal
adam at hupp.org
Thu Sep 28 22:37:19 EDT 2006
I am trying to create an instance of a class for String types. I have
class Foo a where
mkFoo :: a -> String
instance Foo String where
mkFoo x = x
and receive the following error:
Illegal instance declaration for `Foo String'
(The instance type must be of form (T a b c)
where T is not a synonym, and a,b,c are distinct type
In the instance declaration for `Foo String'
Failed, modules loaded: none.
I see that in general I am not able to create an instance of a class for any
type [a], where a is a specific type.
I can work around it by wrapping my Strings in some other data type
but I'd like to understand why it's not possible. Any insight would
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