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Topic: Do we need a When function?
Replies: 5   Last Post: Apr 13, 2014 5:26 AM

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David Bailey

Posts: 714
Registered: 11/7/08
Re: Do we need a When function?
Posted: Apr 13, 2014 5:26 AM
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On 11/04/2014 07:09, Szabolcs Horvát wrote:
> On 2014-3-8, 2:42 , David Bailey wrote:

> Putting aside the question whether this should be a part of the core
> language, I got quite convinced that this is a pretty useful function.
> I am saying this one month after David's original post, so this opinion
> is based on some practical experience. During this month I found myself
> remembering and using this function on a number of occasions. (Not very
> often, but it does keep coming back.)
> Admittedly, in all cases I used it with NumericQ, so I might as well
> have hard-coded NumericQ into it ...
> SetAttributes[whenNumeric, HoldRest]
> whenNumeric[x_?NumericQ, val_] := val
> My point is that it seems that this is going to stay in my personal
> toolbox permanently, it is indeed a useful function.
> To show an example, most recently I used it while experimenting with
> this piecewise function:
> NIntegrate[when[NumericQ[x], First@test[x]], {x, -2, 2}]
> test[] is a function that returns a vector here. Its implementation
> details are irrelevant for my argument.
> Using When here was much easier than defining a separate function only
> to allow NIntegrate to be used on the first component of the vector.
> This When[] function doesn't allow me to do anything I couldn't do
> without it. I wouldn't miss it at all when writing a package. But it
> is very useful for quick-and-dirty or "improvisational" programming,
> i.e. what I do 90% of the time when I use Mathematica interactively. It
> saves me time and effort.
> Szabolcs
> P.S. One thing that bothers me slightly is that I do not yet see clearly
> whether Condition caching might interact badly with this function.

Thanks for your comments - you seem to be the only person to understand
what this function was designed to do!

I did think about simply writing a whenNumeric function, but I felt sure
there would be occasional other interesting applications. For example:


I also felt When would be obviously analogous to If.

Since When can be constructed out of Mathematica code, I reckon caching
will not be a problem - at least not in typical cases.

David Bailey

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