<br><font size=2><tt><br>
Christian Maeder wrote:</tt></font>
<br><font size=2><tt>> Jared Updike wrote:<br>
> > http://www.haskell.org/onlinereport/decls.html#default-decls<br>
> > http://www.haskell.org/tutorial/numbers.html#sect10.4<br>
> <br>
> I still don't see, why it works for show but not for my_show.<br>
> <br>
> > On 1/12/06, Jeff.Harper@handheld.com <Jeff.Harper@handheld.com>
wrote:<br>
> [...]<br>
> >> class (Show a) => My_show a where<br>
> >> my_show :: a -> String<br>
> <br>
> If I let this be<br>
> <br>
> class My_show a where<br>
> my_show :: a -> String<br>
> <br>
> >> instance My_show Int where<br>
> >> my_show a = show a ++ " :: Int"<br>
> >><br>
> >> instance My_show Integer where<br>
> >> my_show a = show a ++ " :: Integer"<br>
> <br>
> What is the difference to the builtin Show class?<br>
</tt></font>
<br><font size=2><tt>I was wondering the same thing myself. Then,
I reread the online Haskell report link. </tt></font>
<br>
<br><font size=2><tt>From http://www.haskell.org/onlinereport/decls.html#default-decls:</tt></font>
<br><font size=2><tt>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: </tt></font>
<br><font size=2><tt> o v appears only in constraints
of the form C v, where C is a class, and </tt></font>
<br><font size=2><tt> o at least one of these classes
is a numeric class, (that is, Num or a subclass of Num), and </tt></font>
<br><font size=2><tt> 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.) <br>
<br>
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. </tt></font>