[Haskell-cafe] Difference between class and instance contexts

[Miguel Mitrofanov]

Try

:t (foo 2, moo 2)

On 3 Aug 2011, at 23:31, Patrick Browne wrote:

> Below are examples of using the sub-class context at class level and at instance level.
>  In this simple case they seem to give the same results
> In general, are there certain situations in which one or the other is preferred?
> Pat
> module CLASS where
> -- class and sub-class
> class Class a where
>  foo :: a -> a
>  foo a = a
> class Class a => SubClass a where
>  moo :: a -> a
>  moo a = foo a
> instance Class Integer where
> instance SubClass Integer where
> *CLASS> :t foo 2
> foo 2 :: forall t. (Class t, Num t) => t
> *CLASS> :t moo 2
> moo 2 :: forall t. (SubClass t, Num t) => t
> module INSTANCE where
> -- Using context at instance level
> -- Is 
> class Class a where
>  foo :: a -> a
>  foo a = a
> class SubClass a where
>  moo :: a -> a
> 
> instance Class Integer where
> instance Class Integer => SubClass Integer where 
>    moo a = foo a
> INSTANCE> :t foo 2
> foo 2 :: forall t. (Class t, Num t) => t
> INSTANCE> :t moo 2
> moo 2 :: forall t. (SubClass t, Num t) => t
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