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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


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