Re: Do we need a When function?

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• Subject: [mg132429] Re: Do we need a When function?
• From: David Bailey <dave at removedbailey.co.uk>
• Date: Thu, 13 Mar 2014 03:10:50 -0400 (EDT)
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```On 10/03/2014 08:37, Itai Seggev wrote:> On Sat, Mar 08, 2014 at 02:42:31AM
>
> I'm not sure what your use case is.  F[x] will of course stay
unevluated until
> x has some value which F knows what to do with...
>
> --
> Itai Seggev
> Mathematica Algorithms R&D
> 217-398-0700
>
A typical use would be an expression like When[NumberQ[x],ToString[x]]

Somehow everyone seems to be missing the point here - of course I can
solve the problem by writing some code - indeed I gave a solution in my
original post - but lots of Mathematica functions can be replaced by
other expressions using more primitive functions, but such functions are
provided for convenience and expressiveness.

Supplying an expression that only evaluates when some condition is true,
seems a pretty useful feature.

The difference between If and When, is obvious:

If[NumberQ[x],f[x]]
Null

When[NumberQ[x],f[x]]

When[NumberQ[x],f[x]]

When delays evaluation until is condition is True.

If you want to hold an expression until (and unless) some condition is
satisfied, When would seem to me to be the most expressive way to write
this.

David Bailey
http://www.dbaileyconsultancy.co.uk

```

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