Type classes vs C++ overloading Re: [Haskell-cafe] Messing around with types [newbie]

Bulat Ziganshin bulat.ziganshin at gmail.com
Thu Jun 21 09:53:54 EDT 2007

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 :)  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:
chapter 7 of GHC user's guide, "functional dependencies"

Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com

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