[Haskell-cafe] instance of String illegal

[Donald Bruce Stewart]

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