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Re: HoldAll/HoldRest

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  Re: HoldAll/HoldRest Heike Gramberg <heike.gramberg@gmail.com> - 2011-05-01 10:20 +0000

#2014 — Re: HoldAll/HoldRest

When you evaluate valueQ[x], x gets evaluated before it's plugged into the function definition, so you're effectively evaluating

If[ValueQ[5],5,False]

which returns False. To get the right behaviour, you could use Unevaluated, e.g.

valueQ[Unevaluated[x]]

You could also add the Attribute HoldAll to valueQ

SetAttributes[valueQ,HoldAll];

valueQ[x]

out: 5

Heike.

On 29 Apr 2011, at 12:33, Scot T. Martin wrote:

> Some kind of issue going on here between the interactions of HoldAll of ValueQ[...] and HoldRest of If[...]. Anyone know the solution so that Out[3] results in True?
> 
> 
> In[1]:= valueQ = If[ValueQ[#], #, False] &
> 
>
> 
> Out[1]= If[ValueQ[#1], #1, False] &
> 
>
> 
> In[2]:= x = 5
> 
>
> 
> Out[2]= 5
> 
>
> 
> In[3]:= valueQ[5]
> 
>
> 
> Out[3]= False

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