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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
Do we need a When function?
Posted: Mar 8, 2014 2:42 AM
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Dear All,

Recently I wanted to create an expression that only evaluated when x was
a number. Obviously, one way to do that is to write something like:


This will stay unevaluated until x is assigned to a number - as required
- but it is very ugly because it makes it hard to understand what is
going on.

More generally, it would be nice to write an expression that will only
evaluate when an expression is true. I solved that by writing a function

SetAttributes[When, HoldAll];
When[cond_, val_] := val /; cond

However, my point is that this construction is sufficiently useful that
it should be built in to Mathematica, and my solution might not be
obvious to all users.

Am I missing a simpler solution?

Do others agree that this is an omission in the language?

David Bailey

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