[Haskell-cafe] Correction: subclasses and classes with same type in instance

Daniel Fischer daniel.is.fischer at googlemail.com
Sun Oct 16 20:35:07 CEST 2011

On Sunday 16 October 2011, 20:03:02, Patrick Browne wrote:
> Hi,
> Does the subclass relation have any meaning when two classes
> have instances with the same type?
> I get the same results from Listing 1 and Listing 2 below.
> Regards,
> Pat

The only effect of a superclass constraint is that you can't make a type an 
instance of the subclass without a superclass instance for the type in 
Usually, a superclass constraint means there is a connection between the 
methods of both classes (like for Eq/Ord), and then it is expected that the 
instances respect that connection, but the compiler can't enforce that.

In your example, the only difference is that with the superclass constraint

foo :: House h => h -> Integer
foo h = addressB h + addressH h

works, while without superclass constraint, foo would need both classes in 
its context.

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