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Re: HoldForm in defined function doesn't seem to work

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99322] Re: HoldForm in defined function doesn't seem to work
  • From: David Bailey <dave at removedbailey.co.uk>
  • Date: Sat, 2 May 2009 06:03:02 -0400 (EDT)
  • References: <gt9aki$q22$1@smc.vnet.net> <gtbuo8$2r4$1@smc.vnet.net> <gtef26$12i$1@smc.vnet.net>

joel.aufgang at gmail.com wrote:
> Wow!  What a great number of responses. Thanks to all of you who
> helped me out with this.
> 
> The SetAttributes[f,HoldAll]  solution was just what I was looking
> for.
> 
> 
> 
> Unfortunately, I'm still stuck on something....  Namely, using this
> with the Map function.
> 
> I'm trying to display the way the notebook front end rewrites the box
> model representation of an expression as it is edited.  I'd like to
> display a sequence of expressions and their ToBoxes representations
> using a SlideView.   So I'm doing something like this:
> 
> ==========================
> ==============
> 
> ShowBoxForms[expr_] := TableForm[ {HoldForm[expr], ToBoxes[HoldForm
> [expr]][[1]], TreeForm[ToBoxes[HoldForm[expr]][[1]]]}];
> EditingSequence[expr_] := SlideView[Map[ShowBoxForms, expr],
> AppearanceElements -> All];
> SetAttributes[ShowBoxForms, HoldAll];
> SetAttributes[EditingSequence, HoldAll];
> {a, a b, a b c, a b c/d} // EditingSequence
> {a, ab, a b, a b c, a b c^2, a b + c^2, a (b + c)^2, a (b + c)^
> (2/3)} // EditingSequence
> {12, 12 34, 12 34/56} // EditingSequence
> 
> ==========================
> ==============
> 
> Unfortunately, even though I am setting the HoldAll attributes on my
> functions, the hold is being removed when the Map function is being
> called. You can see this by looking at the third editing sequence in
> my example, where the numeric expressions are being evaluated before
> being converted to boxes.  "12 34/56 // ShowBoxForms"  Works
> perfectly, but I need it to display the same way when being called
> through EditingSequence
> 
> I've tried setting HoldAll on Map, but that messes everything up even
> more.  Doing this holds the entire list from the argument, which
> causes Map to be applied to the entire list and not each element.  I
> need to be able to use Map while performing a HoldForm on each element
> of the input argument list.
> 
> Any help with this would be greatly appreciated.  And Thanks again for
> the previous help as well.
> 

In my original reply, I suggested that you read up on Evaluated and 
Unevaluated - now you can see why!

SetAttributes[ShowBoxForms,HoldAll];
ShowBoxForms[expr_]:=TableForm[{HoldForm[expr],TreeForm[HoldForm[expr][[1]]]}];

SetAttributes[EditingSequence,HoldAll];
EditingSequence[expr_]:=SlideView[Map[ShowBoxForms,Unevaluated[expr]],AppearanceElements->All];


{a,a b,a b c,a b c/d}//EditingSequence
{a,ab,a b,a b c,a b c^2,a b+c^2,a (b+c)^2,a (b+c)^(2/3)}//EditingSequence
{12,12 34,12 34/56}//EditingSequence

I also removed some spurious calls to ToBoxes, which I assume you did 
not want - put them back if you really  wanted to study the box expressions.

BTW, Never try setting attributes of built-in functions - you will 
disturb other code - including parts of Mathematica itself.

David Bailey
http://www.dbaileyconsultancy.co.uk


David Bailey
http://www.dbaileyconsultancy.co.uk


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