Re: Visibility of value of variable bindings in module

*To*: mathgroup at smc.vnet.net*Subject*: [mg121239] Re: Visibility of value of variable bindings in module*From*: Leonid Shifrin <lshifr at gmail.com>*Date*: Tue, 6 Sep 2011 03:54:51 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*References*: <201109042205.SAA21480@smc.vnet.net>

Hi Chris, As others noted, there is no built-in form that would do that. There were however several attempts to define such an operator within Mathematica. Here is mine: ClearAll[LetL]; SetAttributes[LetL, HoldAll]; LetL /: Verbatim[SetDelayed][lhs_, rhs : HoldPattern[LetL[{__}, _]]] := Block[{With}, Attributes[With] = {HoldAll}; lhs := Evaluate[rhs]]; LetL[{}, expr_] := expr; LetL[{head_}, expr_] := With[{head}, expr]; LetL[{head_, tail__}, expr_] := Block[{With}, Attributes[With] = {HoldAll}; With[{head}, Evaluate[LetL[{tail}, expr]]]]; What it does is to macro-expand the definition into nested `With` at definition-time. Here is a simple example: In[29]:= LetL[{a=2,b=a+3,c=a+b+4},c] Out[29]= 11 Because the definition is expanded not at run-time but at definition-time, LetL preserves the semantics of functions defined with patterns having local variables shared between the body and the condition. In particular, this will work: Clear[f]; f[x_,y_]:=LetL[{xl=x,yl=y+xl+1},xl^2+yl^2/;(xl+yl<15)]; f[x_,y_]:=x+y; ?f Global`f f[x_,y_]:=With[{xl=x},With[{yl=y+xl+1},xl^2+yl^2/;xl+yl<15]] f[x_,y_]:=x+y By nature of the LetL function (macro), it should play well with other (built-in) scoping constructs for issues like variable name conflicts in nested scoping constructs. There won't alas be code highlighting for it. The more extended discussion of this construct lives here: http://stackoverflow.com/questions/5866016/question-on-condition/5869885#5869885 , where I also provided a link to the original Mathgroup thread where this code was first published. I did not do it, but one should be able to write a similar macro for Module. Hope this helps. Regards, Leonid On Mon, Sep 5, 2011 at 2:05 AM, caw <cawright.99 at gmail.com> wrote: > Hi > > Module[{x = Sum[i, {i, Length[{1, 2, 3}]}], y = x + 1}, Print[y]; > Print[x]] > > prints out: > > 1 + x > 6 > > So, in the variable declaration section of the Module statement, the > binding of x to 6 isn't visible to y. > (It's a bit like let and let*, but not) > > Is there a form which will allow the bindings to be visible within the > variable declaration section? > > thanks very much > > chris > >

**References**:**Visibility of value of variable bindings in module***From:*caw <cawright.99@gmail.com>