       Re: Evaluate part of rhs of RuleDelayed

• To: mathgroup at smc.vnet.net
• Subject: [mg91111] Re: Evaluate part of rhs of RuleDelayed
• From: Albert Retey <awnl at gmx-topmail.de>
• Date: Wed, 6 Aug 2008 05:07:16 -0400 (EDT)
• References: <g79a05\$dnl\$1@smc.vnet.net>

```Hi,

> 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 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 be {value :> SomeComplexFunction} ?
>
> Unfortunately, this one
>
> SomeComplexFunction[a_] := Module[{x},
>   x = a*a;
>   {value :> SomeComplexFunction[Evaluate[x + a]]}]
>
> yields {value :> SomeComplexFunction[Evaluate[x\$7421 + 5]]}
>

If I understood correctly what you try to do you should look at With,
which is your friend when working with partially evaluated expressions:

SomeComplexFunction[a_] := Module[{x},
x = a*a;
With[{arg = x + a},
{value :> SomeComplexFunction[arg]}
]
]

On the other hand, I have the feeling that your problem might be solved
in a better way if you give more detailed information on what you are
trying to achieve with SomeComplexFunction...

hth,

albert

```

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