Hi again,
My question about my student's problem surely stirred an interesting and
clarifying discussion. Still, I have a question.
Reconsider the example. There's a data type
data LispList t = Atom t | LispList [LispList t] | Str [Char]
and an instance declaration
instance Show t => Show (LispList t) where
show (Atom t) = show t
show (LispList t) = show t
show (Str t) = show t
So, hypothetically there could have been an additional, overlapping instance
declaration, say
instance Show (LispList Int) where
show (Atom t) = show t
show (LispList t) = show t
show (Str t) = "Blahonga!"
Then we cannot know whether show (Str "HEJ") should yield "HEJ" or
"Blahonga!". However, my student never gave such an overlapping instance,
only the first. So far as I can see there should relly be no ambiguity here!
I'd really like to know what the cause of the problem is.
Björn Lisper
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