Re: [Caml-list] An extended comparative study of language support for generic programming

[Peng Zang <peng.zang@gmail.com>]

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Just read the relevant OCaml section, and I see two things that seem strange:

1) From my understanding, the paper appears to say that OCaml has cannot 
constrain a function parameter with more than one concept, which is untrue.  
Eg.
  # class type foo = object method foo : unit end;;
  # class type bar = object method bar : unit end;;
  # let f ((x:#foo):#bar) = x;;
  val f : (< bar : unit; foo : unit; .. > as 'a) -> 'a = <fun>


2) It also says that constraints specified on type parameters may not 
determine the actual type.  They provide example:

  class type conceptA = object
     method fn1 : int
  end
  let algo (x : #conceptA) : int = x#fn2 + 1

And point out the type of parameter x is not just #conceptA, it also has 
method fn2.  This is true.  But although confusing to new users, is hardly 
limiting.  You can use a sig [A] to ensure the parameter type is correct 
(that's what sigs do after all), or you can simple use the close type and 
then the open [B].

[A]  
  include 
  (struct
     let algo (x : #conceptA) : int = x#fn2 + 1 
   end:sig
     val algo : #conceptA -> int 
   end);;

[B]
let algo (x : conceptA) : int = x#fn1 + 1;;
let algo (x : #conceptA) : int = algo (x:>conceptA);;



Peng
  



On Wednesday 10 February 2010 08:31:00 pm Jon Harrop wrote:
> On Wednesday 10 February 2010 23:00:44 Raoul Duke wrote:
> > > http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.110.122&rep=re
> > >p1 &type=pdf
> > >
> > :-)
> >
> > http://groups.google.com/group/comp.lang.functional/browse_thread/thread/
> >d6 54fedd6efdf753/3ee82770d5e79402#3ee82770d5e79402
>
> See my response there:
>
> http://groups.google.com/group/comp.lang.functional/msg/2cb15a6281087b04
>
> :-)
>
> I was wondering if anyone here was familiar with this work and/or had
> anything to say about their OCaml solutions and discussion?
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