Re: is there something special about the Num instance?

Yes; I had a similar question, and it turns out Num is special, or
rather, pattern matching on integer literals is special.  See the
thread

http://www.nabble.com/Pattern-matching-on-numbers--td20571034.html

The summary is that pattern matching on a literal integer is different
than a regular pattern match; in particular:

> foo 1 = print "one"
> foo _ = print "not one"

turns into

> foo x = if x == fromInteger 1 then "one" else "not one"

whereas

> bar Test = print "Test"
> bar _ = print "Not Test"

turns into

> bar x = case x of { Test -> print "Test" ; _ -> print "Not Test" }

In the former case, the use of (y == fromInteger 1) means that "foo"
works on any argument within the class Num (which requires Eq),
whereas in the latter case, the use of the constructor Test directly
turns into a requirement for a particular type for "bar".

There's no way to get special pattern matching behavior for other
types; this overloading is specific to integer literals.

  -- ryan

On Wed, Dec 3, 2008 at 3:05 PM, Anatoly Yakovenko <aeyakovenko <at> gmail.com> wrote:
> module Test where
> --why does this work:
> data Test = Test
>
> class Foo t where
>   foo :: Num v => t -> v -> IO ()
>
> instance Foo Test where
>   foo _ 1 = print $ "one"
>   foo _ _ = print $ "not one"
>
> --but this doesn't?
>
> class Bar t where
>   bar :: Foo v => t -> v -> IO ()
>
> instance Bar Test where
>   bar _ Test = print $ "test"
>   bar _ _ = print $ "not test"
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