[mercury-users] some [T] ...

[Fergus Henderson]

On 17-Nov-2002, Michael Day <mikeday at yeslogic.com> wrote:
> 
> For the following type:
> 
> :- type foo ---> some [T] foo(T) => bar(T).
> 
> How will it be represented by the compiler?  If not for the existential
> quantifier, presumably it would be a no-tag type. However does the
> existential quantifier mean that values of type foo are represented by a
> pointer to a cell containing two further pointers, one to the T and one to
> the type info for T?

Values of type foo are represented by a pointer to a cell containing two words.
The first word is the "typeclass_info" pointer,  which records information
about the `foo(T)' instance, including the type_info for T and function
pointers for each of the methods of foo(T).  The second word is the
value of type T.  It is not necessarily a pointer.

Our paper on RTTI in Mercury [1] explains more of the details.

> Does the fact that there is a type class constraint 
> on T affect the storage in any way,

Yes...

> for example by requiring an additional 
> pointer to the type class method dictionary? Or is this referenced from 
> the type info?

It's the other way around -- the type info is referenced from the
type class info.

> In this case, I am really looking for a value of a simple type (foo) and a
> simple inst (ground) that I can pass around that actually encapsulates a
> predicate via the type class methods. In some cases, this seems much more
> convenient than passing around a pred(...) and having to get all the modes
> right, which is basically impossible with the current compiler / library.
> 
> However given that T will in many cases be a unit type carrying no 
> information, it would be a shame to take three pointers when one would do. 
> Any tips?

Another alternative is to pass around a func -- if the function has mode
func(in, in, ...) = out then in recent (ROTD) versions of Mercury you
can just use mode `in' or `out'.


References
[1]    "Run time type information in Mercury".
       Tyson Dowd, Zoltan Somogyi, Fergus Henderson, Thomas Conway and
       David Jeffery.
       Proceedings of the International Conference on the Principles and
       Practice of Declarative Programming , Paris, France,
       September/October 1999, Lecture Notes in Computer Science 1702,
       Springer Verlag, Pages 224-243,
<http://www.cs.mu.oz.au/research/mercury/information/papers.html#rtti_ppdp>.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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