Path: utzoo!mnetor!uunet!mcvax!prlb2!kulcs!bimandre
From: bimandre@kulcs.uucp (Andre Marien)
Newsgroups: comp.lang.prolog
Subject: behavior of read/get0 at end_of_file
Message-ID: <1197@kulcs.kulcs.uucp>
Date: 16 Mar 88 10:24:05 GMT
References: 608
Reply-To: bimandre@kulcs.UUCP ()
Organization: Katholieke Universiteit Leuven, Dept. Computer Science
Lines: 50
Keywords: get0 read end_of_file


>	copy_chars :-
>		get0(Char),
>		copy_chars(Char).
>
>	copy_chars(-1) :- !.
>	copy_chars(Char) :-
>		put(Char),
>		copy_chars.

At the benchmark workshop he agitated vividly against the different
behavior of BIM_Prolog in case of end of file.
BIM_Prolog fails when get0 attemps to read past end of file in stead of
returning -1. The same is true for read.
If you write the same program as above with the BIM_prolog convention,
this it what it looks like :

        copy_chars :-
                get0(Char), !,
                put(Char),
                copy_chars.
        copy_chars .

The previous code creates a choicepoint for every character which
get processed. It can be easily avoided :

	copy_chars :- copy_chars_h.
	copy_chars .

	copy_chars_h :- get0(Char), put(Char), copy_chars_h .

Now this looks so obviously better and more readable to me,
that it convinces me we made the better choice.

BTW, if you ever want to convert a program with a different interpretation,
the solution is easy :

/*QP*/read(X) :- /*bim*/read(X), ! .
/*QP*/read(whatever_is_used_to_indicate_end_of_file) .

Of course, as you can verify above, a different coding may very well
produce a better program.

Andre' Marien
B.I.M. Belgium
bimandre@kulcs

Bart Demoen
K.U.Leuven Belgium
bimbart@kulcs