I think this idea is a stairway to duck typing.
I exagerate, of course, but here is my point:

It shouldn't be difficult to make a class:
class HasName a where
  name :: a -> String

The problem is when declaring Foo and Bar instances of HasName, since you have to copy code :
data Foo = Foo String
data Bar = Bar String
instance HasName Foo where
  name (Foo n) = n
instance HasName Bar where
  name (Bar n) = n

I'm sure one can automatize this using TemplateHaskell, but it is not really simple.

What I mean is that GHC should give a means to automatize this kind of situation, for instance:
data Foo = Foo { name :: String }
  (deriving HasName)

Or even:
data Foo = Foo { HasName.name }

Just an idea.

2010/11/10 Ozgur Akgun <ozgurakgun@gmail.com>
I still don't know whether I like this idea or not, but here is the simplest definition I can think of about what it promises.

Using TDNR, it will be possible to write the following code:

data Foo = Foo { name :: String }
data Bar = Bar { name :: String }

getName :: Either Foo Bar -> String
getName (Left  f) = name f
getName (Right b) = name b

However, currently you cannot: "Multiple declarations of 'name'"

There are basically two things you can do to solve this "problem".
 - Use different names for your functions, a la "fooName, barName". This clutters up your code, and sometimes you may not have access to that part of the code.
 - Define these 2 data types in different modules and import qualified, a la "Foo.name, Bar.name". One might still think this clutters up your code.

In any case, as a programmer you need to resolve which function to use depending on types while defining 'getName'. However compiler has enough information to automate this decision for you. This is not a way to do polymorphism, this is merely a way to allow programmers define more than one function with the same name, but different types.

This kinda sounds like what java people think polymorphism is :P

--
Ozgur Akgun

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