[Haskell-cafe] Type Directed Name Resolution

JP Moresmau jpmoresmau at gmail.com
Wed Nov 10 07:56:23 EST 2010

I'm totally out of my depth (coming from Java I'm probably one of these
people with a weird understanding of polymorphism (-:) probably here, but I
agree with Yves. It seems to me that if we accept that, in Ozgur's example,
name can take either a Foo or a Bar, then his getName function could also be
called name and also accept Either Foo Bar. So all of this smells strongly
of type classes. So I think Yves' idea of generating a StringName type class
or something, that all records with a field called name of type String are
automatically instances of, and adding new instances for other types
manually as need be, is good. Crazy, maybe, but good. If it quacks like a
duck and has signed the duck good behavior charter aka type class
definition, it's a duck.


On Wed, Nov 10, 2010 at 1:36 PM, Yves Parès <limestrael at gmail.com> wrote:

> 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 at 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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JP Moresmau
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