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Re: Reevaluation of conditional arguments when the condition has changed


Looking at your example, I noticed that typing foo[a b] directly gives
the right result, but not evaluating expr1. So strange.

On 10/8/05, David Park <djmp at earthlink.net> wrote:
> Dear MathGroup,
>
> Here is a programming problem that stumps me. We start with a condition that is always False.
>
> fooQ[_] := False
>
> Then write a definition that will factor out multipliers of fooQ objects.
>
> foo[a_ b_?fooQ] := a foo[b]
>
> The following does not factor because b, or anything else, fails the test.
>
> expr1 = foo[a b]
> giving: foo[a b]
>
> Now I define b as a fooQ object.
>
> fooQ[b] := True
>
> Now when I enter the same expression the a factors out.
>
> foo[a b]
> giving: a foo[b]
>
> But if I reevaluate expr1 the definition is not applied.
>
> expr1
> giving:  foo[a b]
>
> Why shouldn't I expect that to evaluate and now factor? Is there a proper way to write the definitions so it will evaluate?
>
> David Park
> djmp at earthlink.net
> http://home.earthlink.net/~djmp/
>
>
>


--
Chris Chiasson
http://chrischiasson.com/contact/chris_chiasson


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