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Rather simple function returns curious result. Explanation requested.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg131384] Rather simple function returns curious result. Explanation requested.
  • From: James Stein <mathgroup at stein.org>
  • Date: Tue, 2 Jul 2013 00:48:42 -0400 (EDT)
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  • Delivered-to: l-mathgroup@wolfram.com
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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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