Re: [Haskell-cafe] Re: Strong duck typing / structural subtyping / type class aliases / ??? in Haskell

Yes, the OOHaskell paper blew my mind too, but I still only understood half of it when reading it for the second time (especially the mfix thing was scary :-) Not giving up though ;-) But I wander if the error messages you would get from GHC make it easy to see what is going wrong. It would be great if a library author could somehow customize error reporting too, although I have no idea if that is possible. On Tue, Sep 29, 2009 at 10:03 AM, Alp Mestan wrote:

I had never seen this work, it's just awesome ! And it only needs few Haskell extensions.

Is this work deeply documented somewhere except in research papers ? If not, it could be worth doing, IMO.

On Tue, Sep 29, 2009 at 9:37 AM, wrote:

...

Alp Mestan wrote:

...

Indeed, OCaml has stuctural polymorphism, it's a wonderful feature.

*# let f myobj = myobj#foo "Hi !";; val f : < foo : string -> 'a; .. > -> 'a = <fun>*

And Haskell has that too:

...

-- This is how we define labels. data Field1 deriving Typeable; field1 = proxy::Proxy Field1

-- This is how record selection looks like. foo f = f # field1

The inferred type of foo is

*OCamlTutorial> :t foo foo :: (HasField (Proxy Field1) r v) => r -> v

It doesn't seem too different from the OCaml's type; the type variable r acts as a row type.

The quoted example is the first from many others described in http://darcs.haskell.org/OOHaskell/OCamlTutorial.hs

The file quotes at length OCaml's Object tutorial and then demonstrates how the OCaml code can be written in Haskell. When it comes to objects, structural subtyping, width and depth subtyping, etc., Haskell does not seem to miss match compared to OCaml. In contrast, Haskell has a few advantages when it comes to coercions (one does not have to specify the type to coerce to, as Haskell can figure that out). The other files in that directory give many more example of encoding C++, Eiffel, OCaml patterns.

-- Alp Mestan http://blog.mestan.fr/ http://alp.developpez.com/

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