Re: Using foreach/2

Hello,

since I still don't exactly understand what the copying of the goal by
`foreach/2` leads to, here is a "cleaner" way to do it:

    update_state(Current_state, Next_state) :-
        functor(Current_state, state, Arity),
        functor(Next_state, state, Arity),
        update_args(1, Arity, Current_state, Next_state).

    update_args(N, Arity, Current_state, Next_state) :-
        (   N =< Arity
        -> next_arg_state(Current_state, N, A),
            arg(N, Next_state, A),
            succ(N, N1),
            update_args(N1, Arity, Current_state, Next_state)
        ;   true
        ).

Boris



On Wed, Mar 19, 2014 at 4:18 PM, Boris Vassilev <[email protected]>wrote:

> Hello,
>
> I am having trouble understanding the implications of using `foreach/2`
> from `library(aggregate)`.
>
> Given a "state", represented as a term of arbitrary length, I want to find
> a new state, a term with the same functor, based on some rules. The rules
> themselves use (arbitrary) elements from the current state.
>
> While I don't know the arity of my state functor, it will not change
> during a program run.
>
> The reason why I chose to use a term for representing the state is that I
> don't want to worry about efficiently finding the arguments that a rule
> might need: I use `arg/3` instead (I am under the impression that it has
> efficient and constant time access to term arguments, when its first
> argument is ground).
>
> Now the question: if I would define `update_state/2` like this:
>
>     update_state(Current_state, Next_state) :-
>         functor(Current_state, state, Arity),
>         functor(Next_state, state, Arity),
>         foreach(
>             (   between(1, Arity, N),
>                 next_arg_state(Current_state, N, A) % defined elsewhere....
>             ),
>             arg(N, Next_state, A)
>         ).
>
> Would that be a silly thing to do? How should I do this instead?
> (I know it does the job, but I admit that I don't fully understand the
> implications of `foreach/2` repeatedly copying its second argument `Goal`.
> The obvious other method is to use `=..` and `maplist`.)
>
> Thank you for the help,
> Boris
>
>
>
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