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Re: To be or not to be...

• To: mathgroup at smc.vnet.net
• Subject: [mg62049] Re: To be or not to be...
• From: albert <awnl at arcor.de>
• Date: Thu, 10 Nov 2005 02:50:31 -0500 (EST)
• References: <dksd37$h58$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

Ingolf Dahl wrote:

> 
> To Mathgroup,
> 
> Try the following commands in order:
> 
> b = Sequence[]
> 
> b == Sequence[]
> 
> b != Sequence[]
> 
> b === Sequence[]
> 
> b =!= Sequence[]
> 
> The four last lines all evaluate to "True" for me. Quite amazing!

Was that a question? The reason is that Equal[], SameQ[], UnsameQ[],
Unequal[] all return True if called with zero (or one) arguments, which is
in fact what you are doing, since Sequence[] does what it is meant to do,
no matter whether the head of the expression is List, Equal or UnsameQ. If
you want to test whether something is a Sequence[] or not you will need
another approach, a simple way is to use a head with Attribute
SequenceHold:

SetAttributes[ccc, SequenceHold]
ccc[b] =!= ccc[Sequence[]]
ccc[b] === ccc[Sequence[]]

which will give the results you probably expected...

albert