Christian Maeder wrote: > Jared Updike wrote:
> > http://www.haskell.org/onlinereport/decls.html#default-decls
> > http://www.haskell.org/tutorial/numbers.html#sect10.4
> I still don't see, why it works for show but not for my_show.
> > On 1/12/06, [hidden email] <[hidden email]>
> >> class (Show a) => My_show a where
> >> my_show :: a -> String
> If I let this be
> class My_show a where
> my_show :: a -> String
> >> instance My_show Int where
> >> my_show a = show a ++ " :: Int"
> >> instance My_show Integer where
> >> my_show a = show a ++ " :: Integer"
> What is the difference to the builtin Show class?
I was wondering the same thing myself. Then,
I reread the online Haskell report link.
From http://www.haskell.org/onlinereport/decls.html#default-decls: where n>=0, and each ti must be a type for which
Num ti holds. In situations where an ambiguous type is discovered, an ambiguous
type variable, v, is defaultable if: o v appears only in constraints
of the form C v, where C is a class, and o at least one of these classes
is a numeric class, (that is, Num or a subclass of Num), and o all of these classes are defined
in the Prelude or a standard library (Figures 6.2--6.3, pages -- show the
numeric classes, and Figure 6.1, page , shows the classes defined in the
Notice the last bullet item. class Show is part of the Prelude. However,
class My_show is not part of the prelude. AS far as I can tell, that's
the only reason "my_show 1" produces errors while "show
1" is okay. _______________________________________________
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