Fwd: [Haskell-beginners] instances of different kinds

[Tobias Brandt]

Forgot to reply to the list, sorry.


---------- Forwarded message ----------
From: Tobias Brandt <tob.brandt at googlemail.com>
Date: 27 August 2010 11:14
Subject: Re: [Haskell-beginners] instances of different kinds
To: Greg <greglists at me.com>


On 27 August 2010 10:58, Greg <greglists at me.com> wrote:
> data Foo a = Foo a
>
> class TwoPi a where
>   div2pi :: (Floating b) => a -> b
>

div2pi is polymorphic in a AND b. They are completely independent.

> instance (Floating a) => TwoPi (Foo a) where
>   div2pi (Foo a) = a / (2*pi)
>

but here, div2pi has type Floating a => Foo a -> a.

>
> data Foo a = Foo a
>
> class TwoPi a where
>   div2pi :: a -> Float
>
> instance (Floating b) => TwoPi (Foo b) where
>   div2pi (Foo b) = b / (2*pi)
>

now div2pi has type Floating a :: Foo a -> a, but should
have Floating a :: Foo a -> Float


There are two possible solutions (I can think of).

1. make div2pi less polymorphic:

  class TwoPi a where div2pi :: a -> a

  then your first instance works


2. use associated types:

  class TwoPi a where
      type TwoPiRes a
      div2pi :: a -> TwoPiRes a

  instance Floating a => TwoPi (Foo a) where
      type TwoPiRes (Foo a) = a
      div2pi (Foo a) = a /(2*pi)