[Haskell] Empty instance declaration

Mike Haskel mlh2131 at columbia.edu
Fri Dec 28 11:14:40 EST 2007

You may be interested in a trick relating to type classes which can
address this issue, though it's not the "Haskell way to do it."

You can define Show as a data type, rather than a type class:

type Show a = Either (a -> String) (Int -> a -> String -> String)

show :: Show a -> a -> String
show (Left s) x = s x
show (Right sp) x = sp 0 x ""

showsPrec :: Show a -> Int -> a -> String -> String
showsPrec (Left s) n a str = s a ++ str
showsPrec (Right sp) n a str = sp n a str

A function using this "class" would take it as its first argument:

print :: Show a -> a -> IO ()

The constructors for Show make explicit the two ways to define an
instance.  This technique also has the advantage of allowing multiple,
non-conflicting instance declarations, selectable at runtime.  Using
Show as an example, you might have instances representing both
formatted and unformatted display.  An obvious disadvantage is that
the instance needs a name and gets passed explicitly.


On Thu, Dec 27, 2007 at 04:34:06PM +0000, Hugo Macedo wrote:
> 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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