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restricting and hiding record fields for functors
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Date: -- (:)
From: Jeff Shaw <shawjef3@m...>
Subject: restricting and hiding record fields for functors
Hello. I searched through the Google archives of this list, so I 
apologize if I missed something relevant.

I'm working on a project that uses record types, perhaps

module type Cordinate = sig
  type ordinate
  type point2d = {x : ordinate; y : ordinate;}

  val distance : point2d -> float
end

Now I have a functor over modules that match the signature Coordinate, 
but I want to allow the modules given to the functor to have things in 
the record "point2d" besides just x and y. It seems the types in this 
case are really clear and there shouldn't be any ambiguity (but I'm no 
expert).

As it is now, to use such records I have to entirely hide their contents 
so that the type checker doesn't complain when "point2d" contains extra 
fields. In addition, any fields I need access to in the functor will 
have an associated function for setting and getting the values. For 
instance,

module type Cordinate1 = sig
  type ordinate
  type point2d
  val setx : ordinal -> point2d -> point2d
  val sety : ordinal -> point2d -> point2d
  val makepoint : ordinal -> ordinal -> point2d
  val getx : point2d -> ordinate
  val gety : point2d -> ordinate
  val getpoint : point2d -> ordinate * ordinate

  val distance : point2d -> float
end

This is really quite ugly. I would rather be able to have x and y in the 
signature and get rid of get/set functions. But then you can't have 
extra fields. You can't, for instance, hide them via a signature.

Or perhaps I'm missing some simple technique? Please let me know.

Thanks,
Jeff