🚀 go-pugleaf

In article <467@cmx.npac.syr.edu>, hamid@hilbert.uucp (Hamid Bacha) writes:
> In article <944@cresswell.quintus.UUCP> ok@quintus.UUCP (Richard A. O'Keefe) writes:
> >In article <451@cmx.npac.syr.edu>, hamid@hilbert.uucp (Hamid Bacha) writes:
> >In an Edinburgh-compatible Prolog, "- <number> is a special case; the
> >"-" here is the tightest binding operator and is not disabled by
> >	:- op(0, fy, -).
> >which cancels the usual unary minus.  Thus after doing

> In C-Prolog (version 1.5) unary '-' is defined as op(fx, 500, -) and does
> not bind tighter than operators with higher precedence (lower precedence 
> number). It is treated as a special case in that -1 is negative integer 1 
> instead of -(1). A simple example using C-Prolog proves it:

(1) Surely that's op(500, fx, -).

> C-Prolog version 1.5
> | ?- op(400, xfx, ..).
> | ?- assert(p(-1 .. 2)).
> | ?- p(-X).
> X = 1..2 
> This is clearly different from '..'(-1,2) which Richard claims it should be.
> The same behavior is observed with ALS-Prolog (the other system I have 
> access to in addition to our own Columbus Prolog and MetaProlog).

(2) If this really is the case, it is a bug.  (C Prolog is known to take
    a few short-cuts with operators.)

    The test case which is supposed to make it all clear is
	x^ -2
    If current_op(200, xfy, ^) and current_op(500, fx, -) (as is normally
    the case), this would be illegal, and you should find that
    	x^ -y
    IS illegal.  - <number> being a special case, however, x^ -2 means
    the same as x^ (-2).

(3) Believe it or not, there is method in't.  The idea is to simplify the
    *output* routines.  We would like writeq(X) to be able to say "oh, X
    is a number, I'll just write it", whatever the surrounding context,
    rather than sometimes having to write a negative number as (-4) or
    whatever.

    It could be argued that having -<number> treated specially is a bad
    idea, and that it would be better to have it parsed as an ordinary
    operator and then treated specially when operator and operand are
    combined.  However, in DEC-10 Prolog, all operators can have their
    priority reassigned, and all operators can be cancelled by assigning
    priority 0, so there has to be something special about -<number> or
    it would be possible to get into a state where negative numbers could
    not be entered.

(4) I have just received a copy of ALS Prolog version 1.0 for the
    Macintosh SI and MAc II.  I tried
	write(-(5))
    and writeq(-(5))
    in it.  Both printed "-5", which is quite wrong.  writeq/1 is supposed
    to write something so that it can be read back, so writeq(-(5)) ought
    to write -(5) or (-(5)) or '-'(5) or -((5)) or - (5) or almost anything
    BUT -5.  I think it's fair to call this a bug.

    ALS Prolog fails to report the syntax error in (x^ -y) although this
    is clearly illegal according to the table on p42 and the last two
    paragraphs of section 5.7.  While it does indeed parse -1..2
    (incorrectly) as -(1..2), it parses 1.. -2 as ..(1,-2) which ought to
    be illegal if - doesn't have lower rank than .. .  This inconsistency
    does not seem desirable.

    There is much to admire in ALS Prolog, and I don't want to give anyone
    a bad impression of it.  I just want to point out that its I/O is (in
    this *specific* release) at least as buggy as that of C Prolog, and it
    should not be cited to prove what "Edinburgh syntax" is.
    [That is what the public-domain parser is for.]