Re: "access constant" discriminant

[tmoran@acm.org]

>What is it that you want to do?
   In the case at hand, I'm using the idiom of a limited controlled type
whose initialization/finalization grabs/releases a resource lock, and the
access discriminant tells which resource.
  type Lock_Type(Resource : access Resource_Type) is new
  Ada.Finalization.Limited_Controlled with ...
and later
  Locker : Lock_Type(Resource'unchecked_access);
  ...
But this won't work when Resource is an "in" parameter, eg a function
parameter.  I suppose Ada is telling me that grabbing a lock is
effectively modifying the resource, and thus something that shouldn't be
done when the resource is an "in" parameter, but the question remains, why
can't one say
  type Lock_Type(Resource : access constant Resource_Type) ...
Did the designers foresee and forestall my use with a function?