[mercury-users] Term manipulations in Mercury -- advice requested

[Ralph Becket]

Gregory D. Weber, Tuesday,  1 June 2004:
> Greetings!  I am interested in using Mercury as the programming language
> to develop a first-order rule learning system.  I have done a fair amount
> of Scheme and Common Lisp programming and a wee bit of Prolog.

Good idea!  Let us know how you get on.

> [...]
>
> If I were doing this in Prolog, I would be using functor, arg, and
> "=.." ("univ").  I think I see two possible ways to do it in Mercury.
> 
> 1.  One is to use the facilities of the term and related libraries
> (varset__init, varset__new_named_var, term__atom, term__context,
> term__variable, term__functor, or parser__read_term_from_string, to
> build terms; term__unify and term__substitute, to match terms to
> examples).
> 
> 2.  The other is to use facilities of std_util (functor, construct,
> argument, deconstruct; and for matching, unify or just '=').
> 
> My impression is that (1) is going to be more complicated,
> that it might be more flexible and require less compile-time
> information than (2), and that it might be less efficient than (2).

(1) is fairly efficient: if you're going to be manipulating term
structures, then you should first think of using the term module.  
We use it in many places in the compiler.

(2) is intended for reflective programming where you examine the type
and structure of values at run-time.  You could conceivably use it, but
I wouldn't: I'd imagine a combinators + higher order approach would be
better for your purpose.

(3) You could roll your own specialised term module, but in my
experience this often turns out to be a fair bit of work and doesn't
always pay off.

My advice: pick (1) and explore the other options if you identify a
serious performance issue.

-- Ralph
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