[mercury-users] polymorphism

Tomas By T.By at dcs.shef.ac.uk
Thu Sep 25 13:55:50 AEST 1997

Hi all,

Thanks for the replies Tyson, Fergus. I'm not really sure
you answered my question, because I'm not sure what I'm
trying to ask. :-)

So I'll explain the problem. Any feedback appreciated. At
this point I'm interested in elegant design rather than

I'm writing an XML library. The interface consists of a
document type and procedures to read/write/validate etc.
The problem with XML documents is that apps might want to
treat them either as a stream of elements or as a tree
structure, and that you sometimes want to preserve
whitespace and sometimes not.

So you need four different types:

   stream with whitespace            [ stream(byte) ]
   stream without whitespace         [ stream(word) ]
   tree with whitespace              [€tree(byte) ]
   tree without whitespace           [ tree(word) ]

My solution is to have 'list(item(V,T))' as the document
type, declared as follows.

:- type item(V,T) ---> v(V) ; token(T) ; spec(spec) .

:- type s(_) ---> start_tag(string,list(attribute))
                ; end_tag(string)

:- type t(T) ---> element(string,list(attribute),tree(T))

:- type bs_item == item(s(byte),byte).

:- type stream(T) == list(item(s(T),T)).
:- type tree(T) == list(item(t(T),T)).

:- type byte == char.

:- type word ---> word(string)
                ; number(string)
                ; sym(char)

:- type attribute ---> string - string.

:- type spec ---> ref(ref)
                ; empty_tag(string,list(attribute))
                ; comment(string)
                ; procinstr(string,string)
                ; cdata(string)
                ; doctype(string)

:- type ref ---> amp
               ; lt
               ; gt
               ; apos
               ; quot
               ; char(int,int) % Base, Number
               ; def(string)

"Fergus Henderson" <fjh at cs.mu.oz.au> wrote:
> Why don't you want p/1 to handle things of other types?

I want a procedure 'dump' for example that can handle all the
four types above. The reason I don't want it to handle other
things is I don't want to write the code. :-)

In general, if I have a procedure with several modes, eg:

  :- pred stream_tree(stream(T),tree(T)).
  :- mode stream_tree(in,out) is det.
  :- mode stream_tree(out,in) is det.

but I can't use the same code for both modes, how do I write
the "top-level"?


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