Type inheritance?

[John Griffith]

Just started trying to write a small program in mercury.  Getting the
hang of it, but I'm not quite clear on the type system.

Is there any way to get inheritance of types?

For example, I have some type declarations like:

(1)

:- type ft
   ---> bot
   ;    typ(typ)
   ;    feat(string,ft)
   ;    and(ft,ft)
   ;    or(ft,ft)
   ;    not(ft).

:- type typ
   ---> top
   ;    a
   ;    b.

which is fine, except I'd really like to get rid of the typ/1
constructor in the declaration for ft.  Essentially I want to write
something like this instead:

(2)

:- type ft
   ---> bot
   ;    { typ }
   ;    feat(string,ft)
   ;    and(ft,ft)
   ;    or(ft,ft)
   ;    not(ft).

:- type typ
   ---> top
   ;    a
   ;    b.

and have it be equivalent to having written:

(3)

:- type ft
   ---> bot
   ;    top
   ;    a
   ;    b
   ;    feat(string,ft)
   ;    and(ft,ft)
   ;    or(ft,ft)
   ;    not(ft).

I want to define predicates with arguments of type typ but call them
with arguments of type ft.

Is this possible?  If not, then is there a cost associated with the
constructors or do they get "compiled away"?  In other words, is there
an efficiency difference between (1) and (3)?

-- 
John Griffith
Seminar fuer Sprachwissenschaft, Universitaet Tuebingen
email: <URL:mailto:griffith at sfs.nphil.uni-tuebingen.de>
www: <URL:http://www.sfs.nphil.uni-tuebingen.de/~griffith>