I sent this message yesterday to Bulat but it was intended for the haskel cafe, so I'm resending it here today.

Thank to everyone who answered me privately. Today I'll keep on experimenting and read the reference you gave me.

---------- Forwarded message ----------
From: Cristiano Paris <cristiano.paris.ml@gmail.com>
Date: Jun 21, 2007 6:20 PM
Subject: Re: Type classes vs C++ overloading Re: [Haskell-cafe] Messing around with types [newbie]
To: Bulat Ziganshin <Bulat.Ziganshin@gmail.com >

On 6/21/07, Bulat Ziganshin < bulat.ziganshin@gmail.com> wrote:

Hello Cristiano,

Thursday, June 21, 2007, 4:46:27 PM, you wrote:

> class FooOp a b where
> foo :: a -> b -> IO ()
>
> instance FooOp Int Double where
> foo x y = putStrLn $ (show x) ++ " Double " ++ (show y)

this is rather typical question :)

I knew it was... :D

unlike C++ which resolves any
overloading at COMPILE TIME, selecting among CURRENTLY available
overloaded definitions and complaining only when when this overloading
is ambiguous, type classes are the RUN-TIME overloading mechanism

your definition of partialFoo compiled into code which may be used
with any instance of foo, not only defined in this module. so, it
cannot rely on that first argument of foo is always Int because you may
define other instance of FooOp in other module. "10" is really
constant function of type:

10 :: (Num t) => t

i.e. this function should receive dictionary of class Num in order to
return value of type t (this dictionary contains fromInteger::Integer->t
method which used to convert Integer representation of 10 into type
actually required at this place)

this means that partialFoo should have a method to deduce type of 10
in order to pass it into foo call. Let's consider its type:

partialFoo :: (FooOp t y) => y -> IO ()

when partialFoo is called with *any* argument, there is no way to
deduce type of t from type of y which means that GHC has no way to
determine which type 10 in your example should have. for example, if
you will define

instance FooOp Int32 Double where

anywhere, then call partialFoo (5.0::Double) will become ambiguous

shortly speaking, overloading resolved based on global class
properties, not on the few instances present in current module. OTOH,
you build POLYMORPHIC functions this way while C++ just selects
best-suited variant of overloaded function and hard-code its call

further reading:
http://homepages.inf.ed.ac.uk/wadler/papers/class/class.ps.gz
http://haskell.org/haskellwiki/OOP_vs_type_classes
chapter 7 of GHC user's guide, "functional dependencies"

Mmmmmhhhh... your point is hard to understand for me.

In his message, I can understand Bryan Burgers' point better (thanks Bryan) and I think it's somewhat right even if I don't fully understand the type machinery occuring during ghc compilation (yet).

"From this you can see that 10 is not necessarily an Int, and 5.0 is
not necessarily a Double. So the typechecker does not know, given just
10 and 5.0, which instance of 'foo' to use. But when you explicitly
told the typechecker that 10 is an Int and 5.0 is a Double, then the
type checker was able to choose which instance of 'foo' it should use."

So, let's see if I've understood how ghc works:

1 - It sees 5.0, which belongs to the Fractional class, and so for 10 belonging to the Num class.

2 - It only does have a (FooOp x y) instance of foo where x = Int and y = Double but it can't tell whether 5.0 and 10.0 would fit in the Int and Double types (there's some some of uncertainty here).

3 - Thus, ghci complains.

So far so good. Now consider the following snippet:

foo :: Double -> Double
foo = (+2.0)

bar = foo 5.0

I specified intentionally the type signature of foo. Using the same argument as above, ghci should get stuck in evaluating foo 5.0 as it may not be a Double, but only a Fractional. Surprisingly (at least to me) it works!

So, it seems as if the type of 5.0 was induced by the type system to be Double as foo accepts only Double's.

If I understand well, there's some sort of asymmetry when typechecking a function application (the case of foo 5.0), where the type signature of a function is dominant, and where typechecking an overloaded function application (the original case) since there type inference can't take place as someone could add a new overloading later as Bulat says.

So, I tried to fix my code and I came up with this (partial) solution:

module Main where

class FooOp a b where
foo :: a -> b -> IO ()

instance (Num t) => FooOp t Double where
foo x y = putStrLn $ (show x) ++ " Double " ++ (show y)

partialFoo :: Double -> IO ()
partialFoo = foo 10

bar = partialFoo 5.0

As you can see, I specified that partialFoo does accept Double so the type of 5.0 if induced to be Double by that type signature and the ambiguity disappear (along with relaxing the type of a to be simply a member of the Num class so 10 can fit in anyway).

Problems arise if I add another instance of FooOp where b is Int (i.e. FooOp Int Int):

module Main where

class FooOp a b where
foo :: a -> b -> IO ()

instance (Num t) => FooOp t Double where
foo x y = putStrLn $ (show x) ++ " Double " ++ (show y)

instance (Num t) => FooOp t Int where
foo x y = putStrLn $ (show x) ++ " Int " ++ (show y)

partialFoo = foo 10

bar = partialFoo (5.0::Double)

while I thought could be solved likewise:

module Main where

class FooOp a b where
foo :: a -> b -> IO ()

instance (Num t1, Fractional t2) => FooOp t1 t2 where
foo x y = putStrLn $ (show x) ++ " Double " ++ (show y)

instance (Num t1, Num t2) => FooOp t1 t2 where
foo x y = putStrLn $ (show x) ++ " Int " ++ (show y)

partialFoo = foo 10

bar = partialFoo 5.0

but it didn't work. Here's ghci's complaint:

example.hs:7:0:
    Duplicate instance declarations:
      instance (Num t1, Fractional t2) => FooOp t1 t2
        -- Defined at example.hs:7:0
      instance (Num t1, Num t2) => FooOp t1 t2
        -- Defined at example.hs:10:0
Failed, modules loaded: none.

It seems that Num and Fractional are somewhat related. Any hint? Were my reasonings correct or was it only crap?

Thanks,

Cristiano