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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