Re: Controlling evaluation

*To*: mathgroup at smc.vnet.net*Subject*: [mg83956] Re: Controlling evaluation*From*: Yaroslav Bulatov <yaroslavvb at gmail.com>*Date*: Wed, 5 Dec 2007 07:16:32 -0500 (EST)*References*: <fiji6s$ikm$1@smc.vnet.net> <fim998$run$1@smc.vnet.net>

Using dummy symbol, then replacing it, like you did, is one of the techniques that Robby Villegas' suggested for dealing with the "overarching evaluator". http://library.wolfram.com/conferences/devconf99/villegas/UnevaluatedExpressions/index.html He gives a few other approaches, I think the most general one is to use Replace* to manipulate expressions into the form you want. For instance, the function below uses Extract/Position/ReplacePart to extract expressions matching pattern, evaluate them, then stuff the results back in (*evaluates subexpressions of expr matching pattern,expr's head \ should have HoldAll attribute*) pointEvaluate[expr_, patt_] := Module[{poses, parts}, poses = Position[expr, patt]; (*ReplacePart doesn't work for empty position lists*) If[Length[poses] > 0,(*extract the parts (it also evaluates them), put them back in*)parts = Extract[expr, poses]; ReplacePart[expr, parts, poses, MapIndexed[#2 &, parts]], (*no pattern fits,return expr unchanged*) expr ] ] arg1 = {a, b, c}; arg2 = {d, e, f}; pointEvaluate[Condition[ex, samelistQ[arg1, arg2]], HoldPattern[arg1 | arg2]] pointEvaluate[Hold[{1 + 2, 3*4, 5 + 6}], HoldPattern[_ + _]] My own question for the group -- what is the recommended way to define a function only for arguments with HoldAll head (like Hold, Condition, etc)? Yaroslav On Nov 29, 3:54 am, dh <d... at metrohm.ch> wrote: > Hi, > > Evaluate will evaluate its arguments if it appears as argument in a > > function with one of the Hold.. attributes. Consider: > > Condition[expr, samelistQ[arg1 // Evaluate, arg2 // Evaluate]] > > Condition has the attribute HoldAll. If samelistQ has the asme > > attribute, we therefore need to write Evaluate twice: > > Condition[expr, Evaluate[samelist[Evaluate[arg1],Evaluate[arg1]]] > > hope this helps, Daniel > > magma wrote: > > I am building a package and I need to programmatically construct > > expressions . > > At a certain point I have > > > arg1 = {a, b, c}; (* some list *) > > arg2 = {d, e, f}; (* some other list *) > > samelistQ[list1_List, list2_List] := > > list1 === list2 (* some logic function *) > > > Now I want to construct an expression like > > > Condition[expr, samelistQ[arg1, arg2]] > > > with arg1 and arg2 evaluated, but with samelist[...] not evaluated. > > In other words, I want to obtain exactly this > > > Condition[expr, samelistQ[{a, b, c}, {d, e, f}]] > > > where the 2 args have been evaluated, but samelistQ[...] appears > > always unevaluated > > What is expr is not important, just leave it unassigned. > > The problem is that Condition blocks the evaluation of arg1 and arg2 > > and of samelistQ. > > > Now, doing > > > Condition[expr, samelistQ[arg1 // Evaluate, arg2 // Evaluate]] > > > does not work, because Evaluate does not disappear. > > So what to do? I tried Hold, ReleaseHold, ect, but it didn't work. > > > This is how I solved it > > > Condition @@ List[expr, pp[arg1, arg2]] /. pp -> samelistQ > > > where pp is an unassigned symbol. > > My question: isn't there a better way, using the functions which are > > supposed to control evaluation (Hold, Evaluate, ect) to achieve this > > result? > > > Perhaps a related more general question: > > how do we block > > f[x,y] > > while allowing evaluation of x, and y ? > > Thank you in advance for any comment