Re: [Haskell-cafe] Re: What does the Haskell type system do with "show (1+2)"?

Am Freitag, 13. Januar 2006 11:12 schrieb Christian Maeder:

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.

Says the report: Ambiguities in the class Num are most common, so Haskell provides another way to resolve them---with a default declaration: default (t1 , ... , tn) 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: v appears only in constraints of the form C v, where C is a class, and at least one of these classes is a numeric class, (that is, Num or a subclass of Num), and all of these classes are defined in the Prelude or a standard library (Figures ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ That's the point! ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 6.2--6.3, pages -- show the numeric classes, and Figure 6.1, page , shows the classes defined in the Prelude.) Each defaultable variable is replaced by the first type in the default list that is an instance of all the ambiguous variable's classes. It is a static error if no such type is found.

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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?

It's not declared in the prelude or standard libraries.

Christian

Now the question is, could that restriction be lifted, i.e., would it be possible/worthwhile to let defaulting also take place if user defined classes are involved? Cheers, Daniel