Re: Evaluate part of rhs of RuleDelayed

*To*: mathgroup at smc.vnet.net*Subject*: [mg91095] Re: Evaluate part of rhs of RuleDelayed*From*: David Bailey <dave at Remove_Thisdbailey.co.uk>*Date*: Wed, 6 Aug 2008 05:04:14 -0400 (EDT)*References*: <g79a05$dnl$1@smc.vnet.net>

Eugene Kirpichov wrote: > I am writing a lazy recursive function (actually, a tree of UI widgets > that is too large to be constructed at once). > Consider, for example, the following code: > > SomeComplexFunction[a_] := Module[{x}, > x = a*a; > {value :> SomeComplexFunction[x + a]}] > > The value of SomeComplexFunction[5] is {value :> > SomeComplexFunction[x$7418 + 5]} and seemingly can'd be used for > anything useful. > > Question: > How should I fix the code of SomeComplexFunction so that the value of > SomeComplexFunction[5] be {value :> SomeComplexFunction[30]} ? > > Unfortunately, this one > > SomeComplexFunction[a_] := Module[{x}, > x = a*a; > {value :> SomeComplexFunction[Evaluate[x + a]]}] > > yields {value :> SomeComplexFunction[Evaluate[x$7421 + 5]]} > The With function is extremely useful for injecting values inside code that does not immediately evaluate: SomeComplexFunction[a_]:=Module[{x}, x=a*a; With[{xx=x+a},{value:>SomeComplexFunction[xx]}] ] David Bailey http://www.dbaileyconsultancy.co.uk