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
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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