Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!rochester!bbn.com!nic!bunny!bs30
From: bs30@GTE.COM (Bernard Silver)
Newsgroups: comp.lang.prolog
Subject: Standards question: behavior of arg/3
Keywords: ANSI, standard
Message-ID: <9888@bunny.GTE.COM>
Date: 16 Oct 90 11:04:17 GMT
Organization: GTE Laboratories, Waltham MA
Lines: 55

The latest draft of the ISO Prolog standard has recently come out.
One thing that caught my attention is the specified error behaviour 
for the predicate arg/3.  We (the ANSI group) are interested in
finding out how people use this predicate.

The ISO draft specifies that arg(N,Term,Arg) (with N and Term
instantiated) reports a range error if N is greater than the arity of
Term.   In contrast, many existing Prologs just quietly fail in
this situation.  Which do you prefer?  In particular, do your programs
ever rely on the quiet failure to end a recursion?

Some Prologs ``extend'' arg/3 so that arg(0,Term,Arg) (attempts to)
unifies Arg with the functor of Term.  (Quintus provides a library
predicate to support this use).  Do people use this feature?  The ISO
standard specifies that this use should raise an error, many existing
Prologs either fail quietly or behave as above.

If you believe in quiet failure, when should arg/3 raise an error?

Is arg(-3,Term,Arg) ok?  What about arg(a,Term,Arg)?

We would appreciate any comments!

Newsgroups: comp.lang.prolog
Subject: Standards question: behavior of arg/3
Summary: 
Expires: 
Sender: bsilver@gte.com (Bernard Silver)  
Followup-To: 
Distribution: 
Organization: GTE Laboratories, Waltham MA
Keywords: ANSI, standard

The latest draft of the ISO Prolog standard has recently come out.
One thing that caught my attention is the specified error behaviour 
for the predicate arg/3.  We (the ANSI group) are interested in
finding out how people use this predicate.

The ISO draft specifies that arg(N,Term,Arg) (with N and Term
instantiated) reports a range error if N is greater than the arity of
Term.   In contrast, many existing Prologs just quietly fail in
this situation.  Which do you prefer?  In particular, do your programs
ever rely on the quiet failure to end a recursion?

Some Prologs ``extend'' arg/3 so that arg(0,Term,Arg) (attempts to)
unifies Arg with the functor of Term.  (Quintus provides a library
predicate to support this use).  Do people use this feature?  The ISO
standard specifies that this use should raise an error, many existing
Prologs either fail quietly or behave as above.

If you believe in quiet failure, when should arg/3 raise an error?

Is arg(-3,Term,Arg) ok?  What about arg(a,Term,Arg)?

We would appreciate any comments!