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Re: pattern matching: rules that stop other rules?


Attributes[If]

gives

{HoldRest,Protected}

It's the HoldRest that is causing the behaviour you observe. You can set the 
Attributes of any user-defined function, so you can create behaviours such 
as this yourself.

Steve Luttrell
West Malvern, UK

<croddie at princeton.edu> wrote in message news:ek63v6$913$1 at smc.vnet.net...
> Hello. I've been using Mathamatica for quite a while but without ever
> finding out how the language works fundamentally, which I'm trying to
> do now. I'd be grateful for some help in understanding patterns - it
> seems like a powerful idea to me. There is something I can't work out.
>
> Define a function f [x_]:=0 say
> Now If [ p, 1, f [ 2] ] evaluates to itself. So the rule in the
> definition is not applied to f [ 2 ].
>
> Replace If with some other undefined function, say qwerty, and you get
> qwerty [ p, 1, 0] not surprisingly. And If [ p, 1, f [ 2] ] /. f
> [x_]->0 returns If [ p, 1, 0 ].
>
> Is there a rule associated with If that stops a rule (if that's the
> right expression) from being applied inside it? Can users write such
> rules?
>
> Thanks for any help.
> 



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