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Re: Metastable expressions in Mathematica

• To: mathgroup at smc.vnet.net
• Subject: [mg119725] Re: Metastable expressions in Mathematica
• From: David Bailey <dave at removedbailey.co.uk>
• Date: Sat, 18 Jun 2011 19:57:43 -0400 (EDT)
• References: <201106141014.GAA06248@smc.vnet.net> <ita4j9$mb1$1@smc.vnet.net> <itek4d$s7f$1@smc.vnet.net>

On 17/06/2011 05:10, Richard Fateman wrote:
> On 6/15/2011 4:21 AM, Leonid Shifrin wrote:
>> Hi David,
>>
>> This is a known effect. There is a special command Update, used in such
>> cases to force Mathematica to propagate changes like the one you discuss.
>>
>
> It is, in some views, an error, to define and implement a programming
> language with a semantics that is not deterministic, but depends on (for
> example) on the page in random access memory on which an expression is
> stored, to tell how it will be evaluated.
>
> Or call Update.
>
> I suspect this "feature" is unique to Mathematica among programming
> languages commercially supported.  The fact that this is "known" to some
> people does not make it harmless. The fact that it almost never affects
> people does not make it harmless either.
>
Your final comment is provably false - it is only necessary to look at 
the range of 'features' that are accessible in C, C++, or Fortran by 
accessing outside the bounds of an array! All these languages are, of 
course, commercially supported!

In practice, I find that the exhaustive execution feature of 
Mathematica, is extremely useful, and if it is necessary to leave an 
obscure exception like this, for the sake of efficiency, well so be it.

David Bailey
http://www.dbaileyconsultancy.co.uk

• References:
  • Metastable expressions in Mathematica
    • From: David Bailey <dave@removedbailey.co.uk>