Re: New version, new bugs
- To: mathgroup at smc.vnet.net
- Subject: [mg43175] Re: New version, new bugs
- From: Maxim <dontsendhere@.>
- Date: Sat, 16 Aug 2003 02:53:57 -0400 (EDT)
- References: <bhd9a7$roa$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Andrzej Kozlowski wrote:
> On Tuesday, August 12, 2003, at 10:43 AM, Maxim wrote:
>
> > Also I once again ran into the fact that named and unnamed (Slot[k])
> > function parameters behave differently; consider the following
> > definition:
> >
> > In[1]:=
> > f = #1 /. {x_, y_} :> Refine[Sign[{x, y}], #2] &;
> > f[{a - b, b - a}, a > b]
> >
> > Out[2]=
> > {1, -1}
> >
> > The function f simply returns the signs of the pair of elements under
> > given
> > assumptions, but not in this case:
> >
> > In[3]:=
> > f[{y, x}, y > 0 && x < 0]
> >
> > Out[3]=
> > {-1, 1}
> >
> >
>
> I admit it can be a nuisance but a minor one since to fix it you need
> just to add half a line:
>
> f = #1 /. {x_, y_} :> Refine[Sign[{x, y}], #2] & /. {x ->
> Unique[x], y -> Unique[y]};
>
> In[3]:=
> f[{y,x},y>0&&x<0]
>
> Out[3]=
> {1,-1}
>
> The flexibility of the Mathematica's pattern matching language means
> that one can form a huge number of patterns, many of which the
> designers of the language could not have possibly foreseen. Not
> surprisingly some of them will not work as one might expect.
> On the other hand the same flexibility makes it usually easy to find an
> alternative way to achieve the intended result without the problems
> encountered encountered with the original approach. It seems to me not
> unreasonable to view most of the constructions that do not work and
> which have working alternatives as improperly formed. (The same
> applies, I think, to your earlier HoldPattern[head][body]
> construction. Since what it intends to do is exactly equivalent to
> HoldPattern[head[#]]&[body]-in both cases head is held but body is
> evaluated - and since the latter approach works without any glitches,
> it seems to me reasonable to consider the former to be improperly
> formed and to be avoided).
>
> Andrzej Kozlowski
> Yokohama, Japan
> http://www.mimuw.edu.pl/~akoz/
> http://platon.c.u-tokyo.ac.jp/andrzej/
Using Unique is a very useful suggestion, especially when there are twenty
pattern variables and some of them already have values assigned.
Actually, as I already wrote in my message, Function with named formal
parameters works differently -- which already provides a workaround, but
that's missing the point entirely.
In my opinion, all workarounds always boil down to one thing: 'Don't do
that and the error won't appear.' I think such helpful suggestions aren't
worth much.
As to the claim that 'if it doesn't work as expected, then it's improperly
formed', I don't know how anyone can say that seriously. How stupid the
people who developed Ada and other reliability-focused systems were! They
should have just said, guys, there are millions of programs you can write,
how are we supposed to know how they'll work? If something goes wrong, it's
all your fault!
I'd like to add a couple of words about the Trott-Strzebonski method: I
don't think this is something that should be used as an illustration of the
power of Mathematica language -- it illustrates something else. How is the
user supposed to know that
Hold[x] /. x :> Module[
{},
Random[] /; True
]
gives one answer, but
Hold[x] /. x :> Block[
{},
Random[] /; True
]
works differently? The answer is that nobody can know that without actually
trying it out. I understand this trick can be useful, but I just wouldn't
call an obvious kludge 'a clever idiom'. The words that it was *discovered*
by Trott and Strzebonski are very characteristic -- as though we were
talking about a black box, not something operated by known rules.
Even for something as far from ideal in this aspect as C language (I'm, of
course, not comparing the two languages), it is always mentioned explicitly
in the specifications whenever the result is not well-defined.
Maxim Rytin
m.r at prontomail.com
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