Re: Impressed but confused, gluing a list together...

Ignore the deliberate error "glue" / "join"...the real code says "join".
Sorry.


On 1 October 2013 22:03, emacstheviking <[email protected]> wrote:

> I am dong some gluing of strings together and I wanted an intercalate
> capability like many languagues offer, e.g. "join" in Javascript or
> implode() with PHP and I came up with this:
>
> join([], _, []).
> join([X], _, [X]).
> join([X|Xs], With, [X, With|Acc]) :- glue( Xs, With, Acc).
>
> The great news is that I figured it out first time and it seems to do what
> I need, I also have a flatten() predicate that will produce the final
> output string.
>
> However... I am not sure I truly understand how I did it... if I explain
> my reasoning perhaps somebody can straighten me out?
>
> Rule 1: given an empty list, return one, you can't glue nothing together.
> join([], _, []).
>
>
> Rule 2: If there is a single X in the list, return just that single X.
> That avoid "trailing" glue.
> join([X], _, [X]).
>
> Rule 3: Um...... I used the force I guess.
> join([X|Xs], With, [X, With|Acc]) :- glue( Xs, With, Acc).
>
> Having seen many other things like it I kind of sensed it would do the
> right thing but if somebody could supply me a clear explanation I'd be
> grateful. I kind of know what's going on but I can't express it to my own
> satisfation which is quite annoying right now.
>
> I "intuited" that the rule head should glue the current value of the head
> of the list with the concatenation value *With* plus whatever the
> accumulator has so far and that it should succeed if the same can be done
> with the remainder of the list but somehow I feel that I just saw E.T. ride
> off on the back of a unicorn reading a Marvel comic........
>
> Thanks.
>
>

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