Services & ResourcesWolfram ForumsMathGroup Archive

[Date Index] [Thread Index] [Author Index]

Re: OneIdentity

• To: mathgroup at smc.vnet.net
• Subject: [mg103498] Re: OneIdentity
• From: Chris Osborn <chrisosb at gmail.com>
• Date: Fri, 25 Sep 2009 05:57:26 -0400 (EDT)
• References: <200909240352.XAA13458@smc.vnet.net> <h9fmfl$sgh$1@smc.vnet.net>

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

• Follow-Ups:
  • Re: Re: OneIdentity
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>

• References:
  • OneIdentity
    • From: Chris Osborn <chrisosb@gmail.com>