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[Caml-list] Haskell class type simulation
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Date: 2002-11-21 (21:37)
From: kahl@c...
Subject: Re: [Caml-list] Haskell class type simulation
<johan.baltie@wanadoo.fr> wrote:
 > I'm currently looking for a way to simulate the Haskell "class type" notion 
 > in OCaml i.e. the same kind of :
 > class Eq a where
 >     (==), (/=) :: a -> a -> Bool
 > class (Eq a) => Ord a where
 >     compare                :: a -> a -> Ordering
 >     (<), (<=), (>=), (>)   :: a -> a -> Bool
 >     max, min               :: a -> a -> a
 > For me it's a bit like module signature with an abstract type and an 
 > "extending" semantic and a type constraint.

We looked the other way in:

  author = 	 {Wolfram Kahl and Jan Scheffczyk},
  title = 	 {Named Instances for Haskell Type Classes},
  booktitle = 	 {Proc.\null{} Haskell Workshop 2001},
  year = 	 {2001},
  editor = 	 {Ralf Hinze},
  volume = 	 {59},
  number = 	 {2},
  series = 	 ENTCS,
  note = 	 {See also: \textsf{http://ist.unibw-muenchen.de/Haskell/NamedInstances/}},
  abstract = {Although the functional programming language Haskell
              has a powerful type class system,
              users frequently run into situations
              where they would like to be able to define or adapt
              instances of type classes
              only \emph{after} the remainder of a component has been produced.
              However, Haskell's type class system
              essentially only allows late binding
              of type class constraints on free type variables,
              and not on uses of type class members
              at variable-free types.
              In the current paper we propose a language extension that
              enhances the late binding capabilities of Haskell type classes,
              and provides more flexible means for type class instantiation.
              The latter is achieved via \emph{named instances}
              that do not participate in automatic context reduction,
              but can only be used for late binding.
              By combining this capability with the automatic aspects
              of the Haskell type class system,
              we arrive at an essentially conservative extension
              that greatly improves flexibility of programming
              using type classes and opens up new structuring principles
              for Haskell library design.
              We exemplify our extension
              through the sketch of some applications 
              and show how our approach could be used
              to explain or subsume other language features 
              as for example implicit parameters.
              We present a typed lambda-calculus for our extension
              and provide a working prototype type checker on the basis of 
              Mark Jones' ``Typing Haskell in Haskell''.}

Best regards,


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