Re: Do we need a When function?

• To: mathgroup at smc.vnet.net
• Subject: [mg132448] Re: Do we need a When function?
• From: Itai Seggev <itais at wolfram.com>
• Date: Wed, 19 Mar 2014 04:22:48 -0400 (EDT)
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• References: <20140308074231.0A4506A0B@smc.vnet.net> <lg10pf\$50i\$1@smc.vnet.net> <20140317062651.1B88E69F4@smc.vnet.net>

```David,

Firstly, I would appreciate it if you didn't quote these tiny fragments of my
(or that perhaps you didn't bother to read my email in its entirety).

> On 15/03/2014 07:51, Itai Seggev wrote:
> > If[NumericQ[x], f[x]]   (*are NumberQ, if want an actual number type*)
>
> The difference, which at least Szabolcs appreciates, can be seen in this
> example:
>
> In[3]:= expr = If[NumericQ[x], ToString[x]]
>
> In[4]:= expr /. x -> 2.1
>
> (* The output here is Null *)
>
> In[7]:= SetAttributes[When, HoldAll];
> When[cond_, val_] := val /; cond
>
> In[9]:= expr = When[NumericQ[x], ToString[x]]
>
> Out[9]= When[NumericQ[x], ToString[x]]
>
> In[10]:= expr /. x -> 2.1
>
> Out[10]= "2.1"
>
> The effect of When is to delay its evaluation until something is true -
> which can be quite useful!

But on the other hand you have these behaviors:

In[59]:= ClearAll[When]
SetAttributes[When, HoldAllComplete];
When[cond_, val_] := val /; cond

In[62]:= expr = When[NumericQ[x], ToString[x]]

Out[62]= When[NumericQ[x], ToString[x]]

In[64]:= Block[{x = 2.1}, expr ]

Out[64]= When[NumericQ[x], ToString[x]]

In[65]:= x = 2.1; expr

Out[65]= When[NumericQ[x], ToString[x]]

Are these useful?

```

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