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Re: Rather simple function returns curious result. Explanation

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  • Subject: [mg131387] Re: Rather simple function returns curious result. Explanation
  • From: Fred Simons <f.h.simons at tue.nl>
  • Date: Wed, 3 Jul 2013 04:55:25 -0400 (EDT)
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Hi James,

The problem is in List___.  In this way, you use the Mathematica name 
List as a dummy name for passing the arguments into the function body. 
If you use the correct syntax ___List (any number of arguments, all with 
head List), the problem is over.

Regards,

Fred Simons
Eindhoven University of Technology

Op 2-7-2013 6:48, James Stein schreef:
> For reasons that escape me, the simple function below fails to return
> an empty List when n==2; Is this a bug? If not, what is the
> explanation? If the return value for n==2 is changed to an integer or
> a string, the function behaves as expected.(Of course, this bizarre
> function is the result of simplifying a more reasonable one.)
>
> Clear[ f ];
> f [run : { List___ } ] := Module [ { n },
>     n = run // Length;
>     If [ n != 2, Return [ n ] ];
>     Module [ { } ,
>      { } (* Return Empty List if n==2 *)
>      ]
>     ] ;
> f [ { } ]
> f [ { { } } ]
> f [ { { }, { } } ]
> f [ { { }, { }, { } } ]
> f [ { { }, { }, { }, { } } ]
>
> When I run the above, I get these four outputs:
> 0
> 1
> Sequence[{}, {}][]
> 3
> 4
>
>




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