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Re: Rather simple function returns curious result. Explanation

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  • Subject: [mg131398] Re: Rather simple function returns curious result. Explanation
  • From: Daniel <dosadchy at its.jnj.com>
  • Date: Wed, 3 Jul 2013 04:59:06 -0400 (EDT)
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You probably intended to write (and it works as expected)
> f [run : { ___List } ]
which means that you expect a list of zero or more lists

instead of 
> f [run : { List___ } ]
in which you expect a list of expressions, the Sequence of which you name List (and that is a bad idea)

> 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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