[mercury-users] Field Update and Field Selection.

[Fergus Henderson]

On 11-Oct-2002, Noel  Pinto <cool4life at rediffmail.com> wrote:
> :- pred get_next(string::in, bool::out, io__state::di, 
> io__state::uo) is det.
> get_next(String, More) -->
> 	io__read_word(get_a), { More = yes},
> 		if get_a = ok then set_a = get_a else More = no,
> 	io__read_word(get_b), {More =yes},
> 		if get_b = ok then set_b = get_b else More = no,
> 	io__read_word(get_c), {More = yes},
> 		if get_c = ok then set_c = get_c else More = no.

I have no idea what you are trying to do here.
What did you expect "io__read_word(get_a)" and "set_a = get_a" to do??

Note that "=" in Mercury is unification, not assignment.
If "More" has the value "yes", then the goal "More = no"
will just fail, it won't assign a new value to "More".
Since this procedure is declared as "det", it is not
allowed to fail, so this failure would lead to a determinism error.

> Here are the errors:-
> upd.m:046: Syntax error at token 'if': unexpected token at start 
> of (sub)term.

That's because the "if" statement needs to be enclosed in parentheses.

> upd.m:041: In clause for predicate `upd:main/2':
> upd.m:041:   in argument 1 of call to predicate `do_while/3':
> upd.m:041:   error: undefined symbol `getnext/0'.

That's because you mispelt "get_next" as "getnext".

> upd.m:041: In clause for predicate `upd:main/2':
> upd.m:041:   error: undefined predicate `do_while/3'.

That's because you didn't import the "std_util" standard library module.

> upd.m:043: Error: no clauses for predicate `upd:get_next/4'.

This is a spurious flow-on error caused by the syntax error on line 46.
It will go away once you fix the syntax error.

-- 
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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