adam:
I am trying to create an instance of a class for String types. I have the following:
class Foo a where mkFoo :: a -> String
instance Foo String where mkFoo x = x
and receive the following error:
test.hs:9:0: 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 variables) 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.
Ok.
class Foo a where mkFoo :: a -> String
N.B. The instance type must be of form (T a b c), where T is not a synonym. One portable solution, introduce a newtype.
newtype Strung = S String instance Foo Strung where mkFoo (S x) = x
*Main> mkFoo (S "foo") "foo" Alternatively, use -fglasgow-exts :)
instance Foo String where mkFoo = id
$ ghci -fglasgow-exts A.hs *Main> mkFoo "foo" "foo" -- Don