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

[Jared Updike]

http://www.haskell.org/onlinereport/decls.html#default-decls
http://www.haskell.org/tutorial/numbers.html#sect10.4

On 1/12/06, Jeff.Harper at handheld.com <Jeff.Harper at handheld.com> wrote:
>
> What does the Haskell type system do with expressions such as these . . . ?
>    show 1
>    show (1+2)
>
> The type of the subexpressions "1" and "1+2" are "ambiguous" since they have
> type "(Num a) => a".  I'm under the assumption before "1+2" is evaluated,
> the "1" and "2" must be coerced into a "concrete" type such as Int, Integer,
> Double, etc, and before "show 1" is evaluated, the "1" must be coerced into
> a "concrete" type.  Is my assumption correct?  If so, how does Haskell know
> into which type to coerce the subexpressions?
>
> If I try to write a new function, "my_show", which converts an expression
> into a string representation that includes type information, I run into
> errors with expressions like "show 1" and "show (1+2)" because of the type
> ambiguity.
>
> class (Show a) => 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"
>
> I can avoid the errors if I change it to "my_show (1::Int)" or "my_show
> ((1+2)::Int).  I'm wondering what the difference is between, my_show and
> Haskell's built-in show that causes my_show to produce an error message when
> it is used with ambiguous types, but Haskell's show works okay with
> ambiguous types.
>
>
>
>
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>


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