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, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote:
> [...]
> >> 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 Prelude.)
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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