[Haskell-cafe] Missing a "Deriving"?

[nowgate at yahoo.com]

Hi Ryan,

Is there something missing or mislabeled in your post, because I don't see any definition of toDynamic.

Michael

--- On Sun, 5/31/09, Ryan Ingram <ryani.spam at gmail.com> wrote:

From: Ryan Ingram <ryani.spam at gmail.com>
Subject: Re: [Haskell-cafe] Missing a "Deriving"?
To: "David Menendez" <dave at zednenem.com>
Cc: "michael rice" <nowgate at yahoo.com>, haskell-cafe at haskell.org, "Miguel Mitrofanov" <miguelimo38 at yandex.ru>
Date: Sunday, May 31, 2009, 12:45 AM

On Sat, May 30, 2009 at 6:33 PM, David Menendez <dave at zednenem.com>
 wrote:
> *Main> :t searchAll
> searchAll :: (Computation c) => Graph t t1 -> Int -> Int -> c [Int]
>
> The way searchAll is written, the choice of which functions to use
> depends on the type variable c. That's determined by the calling
> context of searchAll, which is why you need to provide a type
> signature when using it at the GHCi command line.

This is actually one of the most interesting and important things to
"get" about typeclasses; it's not *just* like an interface, because
the instance type can appear in the result of a function and *not* in
the arguments at all.

In contrast, in Java/C++, the method to use is always chosen by the
object being called; one could argue that all of COM is an attempt to
get around this problem.

Some examples:

> fromInteger :: Num a => Integer -> a
> fromDynamic :: Typeable a => Dynamic -> Maybe
 a

In both of these cases, the choice of which instance to use is made by
the caller, and often automatically by type inference:

> test x = case fromDynamic x of
>    Just s -> s == "hello"
>    Nothing -> False

Now (test $ toDynamic "hello") = True, but (test $ toDynamic 'a') =
False.  Notice that I never directly specified what type "fromDynamic"
should return; but the case statement forces it to return Maybe
String, since I compare (s == "hello")

  -- ryan



      
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