[Haskell] Empty instance declaration
hmacedo at di.uminho.pt
Thu Dec 27 11:34:06 EST 2007
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
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,
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