[Haskell] Empty instance declaration

[Hugo Macedo]

Dear all

A student from a beginners course on Functional Programming came to me with
a problem, he declared a type like:

data WeekDay = Mon | Tue | Fri -- ...

He had forgot to complete a Show instance definition.

instance Show WeekDay where
(empty)

Then he complained about getting  *** Exception: stack overflow when he
tried to invoke some function on that type.

After checking the Haskell98 grammar I found out that this is allowed
syntactically and probably semantically too. Is there any reason to do that?


Now I'm wondering: Shouldn't a class be an warranty of a well defined
interface? Thus forcing the user to define the show function or showPrec? Am
I missing any point?

My best regards,
Hugo
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