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Re: Mathematica daily WTF

  • To: mathgroup at smc.vnet.net
  • Subject: [mg114994] Re: Mathematica daily WTF
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Tue, 28 Dec 2010 06:47:26 -0500 (EST)

Copying attributes seems to be simple (or am I missing something?):

   mysin = Sin;
   Attributes[mysin]
{}
   SetAttributes[mysin, Attributes[Sin]]
   Attributes[mysin]
{Listable, NumericFunction, Protected}

But subsequent removal of attributes can require another step, as here, 
where Protected is one of the attributes.

   Unprotect[mysin]
{mysin}
   Remove[mysin]

On 12/26/2010 4:02 AM, kj wrote:
> In<if1o3h$p60$1 at smc.vnet.net>  Bill Rowe<readnews at sbcglobal.net>  writes:
>
>> Adding more layers of abstraction won't change this behavior.
>> Additional layers of abstraction simply obscures the fact that
>> Set doesn't pass on attributes.
>
> Our brains, yours and mine, are clearly deformed in very different
> ways...
>
> Mine has been deformed by programming languages other than Mathematica,
> in which assignment does transfer everything, and where one common
> way to exploit localization in a dynamic scope involves starting
> out by copying the global value to the local value (which is
> subsequently modified, with the knowledge that all these modifications
> will be effectively rolled back at the end of the dynamic scoping
> construct).  Hence, it is definitely surprising to someone with
> this experience to learn that one can't do this with Mathematica's
> so-called "dynamic scoping" construct, namely Block.  Why?  Well,
> it's ***logically obvious***, isn't it:  Mathematica's "initialization"
> doesn't transfer most of a symbol's values to the LHS of the
> "assignment".  (Why would anyone be surprised, I wonder...)  In
> fact, this limitation severely reduces the usefulness of dynamic
> scoping in Mathematica to the case where localization equals "clean
> slate".
>
> The fact that these limitations of Mathematica's so-called "assignment"
> are true not only of the initialization clause of a Block statement,
> but actually true *everywhere* in Mathematica is only more WTF-worthy,
> not less.
>
> In fact, I don't know of any simple way to fully copy one variable
> to another in Mathematica.  It can be done, I'm sure, but not
> succinctly, AFAIK.
>
> Here's another hilarious bit:
>
> Block[{Sin = Sin},
>   Print[N[Sin[Pi/4]]];
>   Print[Messages[Sin]];
> ]
>
> Sin[0.785398]
>
> {HoldPattern[Sin::usage]:>Sin[z] gives the sine of z. }
>
> Indeed, as described already, assigning Sin to Sin in the Block's
> initialization clause gets me nothing useful, the local Sin is dead
> as a doornail, and yet it still carries around its now totally
> inaccurate, hence useless, usage message!  That's touching.  (To
> be precise, the initialization shown above is not responsible for
> the persistence of the messages; one would get exactly the same
> results if the initialization clause had been {Sin} instead of
> {Sin=Sin}.)  The utility of keeping the messages around when
> everything else about the symbol has been stripped away is beyond
> me.
>
>
> Bottom-line: the meanings that Mathematica puts on of bread-and-butter
> programming terms like "="/"assignment", "value", and "dynamic
> scope" are entirely sui-generis, and therefore the Mathematica
> documentation should at least show the user the courtesy of making
> this perfectly clear, rather than let him/her figure all this out
> through hard knocks (in the form of "things that *should* work but
> don't").
>
> ~kj
>

-- 
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305


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