Re: Using Defer on Dynamic Expressions?

*To*: mathgroup at smc.vnet.net*Subject*: [mg129748] Re: Using Defer on Dynamic Expressions?*From*: John Fultz <jfultz at wolfram.com>*Date*: Sun, 10 Feb 2013 03:24:54 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net*References*: <20130209054250.CD8036929@smc.vnet.net>

As you may know, when you do an evaluation in Mathematica, there are two steps for preparing that evaluation to return as an output. First, the evaluation is actually performed. Then the evaluation is "typeset", which is to say the kernel runs MakeBoxes on the result of the evaluation and sends the boxes back to the front end. There are some functions which don't do anything particularly interesting at evaluation time, but do something interesting as a result of typesetting. For example, Grid[] is very boring at evaluation time. When you evaluate it, it evaluates its arguments, but the Grid[] itself still remains as something of an inert wrapper. But when you typeset it, you suddenly get this new and interesting behavior where you get a grid onscreen. Since CellPrint of ExpressionCell is doing typesetting, that's the kind of behavior you're getting. E.g., CellPrint[ExpressionCell[Grid[{{1, 2}, {3, 4}}]]] What may not be obvious is that things like Dynamic, DynamicModule, and Manipulate are *exactly* like Grid in this regard. The evaluation properties of Dynamic and Manipulate vary slightly since they hold their arguments. But otherwise, these are functions which essentially do nothing interesting until you typeset them. And since we know that CellPrint of ExpressionCell does typesetting, that explains the behavior you're seeing. So Defer isn't helpful, since all Defer does is prevent evaluation, not typesetting. There's no documented way to do anything different. Even if you call MakeBoxes or ToBoxes by hand, that won't help, as they're also going to just typeset the result. By any documented means, it looks hopeless. But Mathematica does know how to do this. If you use the Cell->Convert To->StandardForm menu command on a cell, it doesn't turn all Grids into GridBoxes or all Manipulates into live active Manipulates. So it's definitely possible. Which means, to answer this question, I'm going to have to go way off the rails into undocumented (and unsupported) territory. So, the best answer I can give to your question involves calling ToBoxes under controlled circumstances. And the controlled circumstance involves setting the following internal variable to True: BoxForm`$UseTextFormattingWhenEvaluating This variable is looked at by the internal typesetting system to determine whether to format things like Grid and Manipulate textually or as GridBoxes and interactive interfaces. Setting it to True gets the typesetting we want. So let's use that to define a new CellPrint-ing function: SetAttributes[newCellPrint, HoldAll]; newCellPrint[cell_] := CellPrint[ ExpressionCell[ Block[{BoxForm`$UseTextFormattingWhenEvaluating = True}, RawBoxes[MakeBoxes[cell]]], "Input"]] Notice I'm not using Defer here because MakeBoxes is HoldAll and our new function is HoldAll, so Defer is no longer necessary. But now I've locally changed the behavior of our MakeBoxes call by changing BoxForm`$UseTextFormattingWhenEvaluating in the Block[] to get the behavior your'e looking for. Although this is undocumented and unsupported, it works from at least v7 (and probably v6, but I didn't test it) to v9, and I don't anticipate any reason why this would change in the foreseeable future. John Fultz jfultz at wolfram.com User Interface Group Wolfram Research, Inc. On Feb 8, 2013, at 11:42 PM, W Craig Carter <ccarter at MIT.EDU> wrote: > Hello Mathgroup, > I am doing a little notebook programming and am stuck on something: > > Note that these two commands differ in their behavior: > > CellPrint[ > ExpressionCell[ > Defer[ > Table[x, {x, 0, 10, 1}] > ], > "Input"] > ] > > CellPrint[ > ExpressionCell[ > Defer[ > Manipulate[x, {x, 0, 10, 1}] > ], > "Input"] > ] > > The front end seems to ignore the Defer request when a command would > have dynamic output. Is there any way to get the same behavior (i.e., > that for Table) as for Manipulate? > > Thanks, Craig > > > W Craig Carter > Professor of Materials Science, MIT

**References**:**Using Defer on Dynamic Expressions?***From:*W Craig Carter <ccarter@MIT.EDU>