Re: OneIdentity
- To: mathgroup at smc.vnet.net
- Subject: [mg103522] Re: OneIdentity
- From: Chris Osborn <chrisosb at gmail.com>
- Date: Sat, 26 Sep 2009 06:13:47 -0400 (EDT)
- References: <200909240352.XAA13458@smc.vnet.net> <h9fmfl$sgh$1@smc.vnet.net>
On Sep 25, 6:51 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 ?
>
> Andrzej Kozlowski
Thanks, these help.
I guess I was not searching "the archives" then :-)
Chris
- References:
- OneIdentity
- From: Chris Osborn <chrisosb@gmail.com>
- OneIdentity