Re: Re: OneIdentity
- To: mathgroup at smc.vnet.net
- Subject: [mg103531] Re: [mg103515] Re: OneIdentity
- From: DrMajorBob <btreat1 at austin.rr.com>
- Date: Sun, 27 Sep 2009 07:30:23 -0400 (EDT)
- References: <200909240352.XAA13458@smc.vnet.net> <h9fmfl$sgh$1@smc.vnet.net>
- Reply-to: drmajorbob at yahoo.com
And so it remains, as always, clear as mud.
But sometimes useful.
Bobby
On Sat, 26 Sep 2009 05:12:31 -0500, Mariano Suárez-Alvarez
<mariano.suarezalvarez at gmail.com> wrote:
> On Sep 25, 8:51 am, Andrzej Kozlowski <a... at mimuw.edu.pl> wrote:
>> On 25 Sep 2009, at 18:57, Chris Osborn wrote:
>>
>>
>>
>> > On Sep 24, 6:51 am, Andrzej Kozlowski <a... at mimuw.edu.pl> wrote:
>> >> The reason is that OneIdentity does not actually mean what the
>> >> documentation seems to imply. What it does exactly would take too
>> >> long
>> >> to explain and it has been already done a number of times on this
>> >> forum . Here is a very brief and basic "explanation" with two
>> >> examples
>> >> (if you search the archives for Flat, OneIdentity adding perhaps the
>> >> names Allan Hayes or Hartmut Wolf you will find a lot more on this
>> >> topic).
>>
>> >> When used alone, without the Flat attribute, it is only significant =
>
>> >> in
>> >> connection with Default values. Here is an example.
>>
>> >> Remove[plus];
>> >> MatchQ[1, plus[_ : 0]]
>>
>> >> False
>>
>> >> Remove[plus];
>> >> SetAttributes[plus, {OneIdentity}];
>> >> MatchQ[1, plus[_ : 0]]
>>
>> >> True
>>
>> >> When used together with the attribute Flat it has a rather different
>> >> significance:
>>
>> >> Remove[f]
>> >> SetAttributes[f, Flat];
>> >> MatchQ[f["a", "b", "c", "d"], f[__, _String, __]]
>>
>> >> False
>>
>> >> but
>>
>> >> Remove[f]
>> >> SetAttributes[f, {Flat, OneIdentity}];
>> >> MatchQ[f["a", "b", "c", "d"], f[__, _String, __]]
>>
>> >> True
>>
>> >> The reason why the first match did not work is that an additional f
>> >> was wrapped around the arguments. Having the OneIdentity attribute
>> >> prevents this form happening.
>>
>> >> Andrzej Kozlowski
>>
>> >> On 24 Sep 2009, at 12:52, Chris Osborn wrote:
>>
>> >>> I do not understand why I get "False" instead of "True" here:
>>
>> >>> In[773]:= ClearAll[key];
>> >>> SetAttributes[key, OneIdentity];
>>
>> >>> MatchQ[13, key[_]]
>>
>> >>> Out[775]= False
>>
>> >>> According to the docs:
>>
>> >>> OneIdentity
>> >>> is an attribute that can be assigned to a symbol f to indicate
>> >>> that f
>> >>> [x], f[f[x]], etc. are all equivalent to x for the purpose of
>> >>> pattern
>> >>> matching.
>>
>> >>> Thanks in advance,
>> >>> Chris
>>
>> > Thanks Andrzej,
>>
>> > Your post is helpful. I find the following behavior amusing:
>>
>> > In[1438]:= MatchQ[monkeyfish[turtle], Plus[_ : car]]
>>
>> > Out[1438]= True
>>
>> > Where none of my cute symbols have any defs associated with them.
>>
>> > I indeed searched for OneIdentity before I posted my questions, but
>> > for some reason 0 results were returned.
>> > Now when I do a search, I get this one thread back.
>>
>> > Thanks,
>> > Chris
>>
>> How about this:
>>
>> http://forums.wolfram.com/mathgroup/archive/2000/Jan/msg00202.html
>>
>> this
>>
>> http://forums.wolfram.com/mathgroup/archive/2000/Jan/msg00254.html
>>
>> this
>>
>> http://forums.wolfram.com/mathgroup/archive/2000/Jan/msg00207.html
>>
>> and quite a few others in these and other threads ?
>
> The search facility in Google Groups has been having issues lately...
>
> -- m
>
--
DrMajorBob at yahoo.com
- References:
- OneIdentity
- From: Chris Osborn <chrisosb@gmail.com>
- OneIdentity