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Re: More /.{I->-1} craziness
*To*: mathgroup at smc.vnet.net
*Subject*: [mg106203] Re: More /.{I->-1} craziness
*From*: Richard Fateman <fateman at cs.berkeley.edu>
*Date*: Tue, 5 Jan 2010 01:43:50 -0500 (EST)
*References*: <200912300915.EAA17299@smc.vnet.net> <hhhmn8$o9t$1@smc.vnet.net> <hhkjc4$5ag$1@smc.vnet.net> <hhpl4j$9mf$1@smc.vnet.net>
David Bailey wrote:
> Richard Fateman wrote:
>> Leonid Shifrin wrote:
>> ...
>>
>>> I think that there are not many more objects in Mathematica which are as
>>> tricky as <I> or Infinity in terms of pattern-matching.
>>
>> I agree.
>> That's why it can be fixed.
>>
>> Here's a beginning of a short list for the "we're not just talking
>> syntactic replacement-- version of substitution":
>>
>> If the user says -i --> i, then do Complex[a_,-b_] -> Complex[a,b].
>> If the user says x^2 --> y, then do x^(-2)-> 1/y also.
>>
>> I assume this list can be enlarged somewhat, and could even be left
>> open-ended by user option of some sort. [e.g. should x^2--> y also
>> change x^3 to x*y? or to y^(3/2) or ....]
>>
>> ....
>>
>>> It would perhaps be nice if such
>>> cases were more systematically documented, but they have nothing to do with
>>> bugs,
>> You are right if you mean "bug in Mathematica implementation of
>> intended design" (this is not such a bug).
>>
>> But there is another concept:
>> "bug in Mathematica design, contrary to reasonable mathematical expectation"
>> (this IS such a bug).
>>
> Would you really want to use a system which had been arbitrarily hacked
> by well meaning individuals to create exceptions that suited their taste?
Are you describing Mathematica here? It seems to me that it is an
exception that Head[a+b*I] is Plus, but Head[3+4I] is Complex.
>
> Perhaps the documentation should make it clearer which operations are
> supposed to always perform mathematically valid operations, and which
> are meant to operate on the structure of expressions, oblivious to their
> meaning.
You assume that people read the documentation. This is provably false.
I think this is an important distinction. If you pass an
> expression to Integrate (say), you expect to get the integral of that
> expression back as a result (possibly in symbolic form still involving
> an integral sign) - anything else is a bug.
Or an error message like "division by zero" or "out of memory".
However, other operations -
> such as ReplaceAll - are not *defined* mathematically, they are defined
> structurally, and as I pointed out earlier, can be used to produce
> explicitly invalid expression transformations.
>
This is obviously not the view of the person who initially posted this
question.
> Are you really saying that Mathematica doesn't need structural
> operations?
No.
Just that people want other kinds of operations for substitution. This
should be obvious from the subject line, I -> -I etc.
If it does, why pick on ReplaceAll?
Because this is enshrined in the syntax of the user language, and
APPEARS (falsely, as it turns out) to be a mathematical operation.
RJF
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