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Re: Unevaluated

  • To: mathgroup at
  • Subject: [mg44489] Re: Unevaluated
  • From: Jens-Peer Kuska <kuska at>
  • Date: Wed, 12 Nov 2003 08:01:37 -0500 (EST)
  • Organization: Universitaet Leipzig
  • References: <bopcq6$op3$>
  • Reply-to: kuska at
  • Sender: owner-wri-mathgroup at


a) use ordinary Message[] that you can individual turn on and off
   for that

b) Unevaluated[] prevent the evaluation in a function call of 
   a function that would evaluate its arguments. If you whant to
   write a function that does not evaluate its arguemtns it must have
   attribute HoldAll. 

You mean

SetAttributes[tPrint, HoldAll]

tPrint[seq___] /; Testing := Print[seq]
tPrint[___] := Null


Hans-Peter Kunzle wrote:
> Hello,
> I was trying to make a function like 'tPrint[x___]' that would act
> like Print if some global variable Testing was True and do nothing
> if Testing was False. This would allow me to simply add or delete
> the 't' to individually turn on or off some tracing. It would
> be easier than to comment or uncomment the Print statements each
> time.
> But when I tried
>         tPrint[Unevaluated[x___]]:= If[Testing,Print[x]]
> I found, using TracePrint, that 'x' gets evaluated each time whether
> or not Testing is True.
> Maybe I do not understand properly how Unevaluated operates.
> Is there a way to do what I want?
> Any help is appreciated.
> Hans

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