[Haskell-cafe] Re: Haskell-Cafe Digest, Vol 29, Issue 36

Christian Maeder wrote:

Jared Updike wrote:

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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.

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On 1/12/06, Jeff.Harper@handheld.com wrote: [...]

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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

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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?

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

I was wondering the same thing myself. Then, I reread the online Haskell report link. 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.