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Re: x=2;Composition[f,FindMinimum][x+1,{x,a}]

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  • Subject: [mg68327] Re: [mg68282] x=2;Composition[f,FindMinimum][x+1,{x,a}]
  • From: "Carl K. Woll" <carlw at>
  • Date: Tue, 1 Aug 2006 06:59:58 -0400 (EDT)
  • References: <> <> <>
  • Sender: owner-wri-mathgroup at

Chris Chiasson wrote:
> The crash confused matters for me. Thank you for clearing that up. Now
> that the source is known, perhaps someone could crack the "original"
> question I am trying to solve:
> x=2;{Composition[Hold][x+1,{x,a}],Composition[Hold,Hold][x+1,{x,a}]}
> produces
> {Hold[x+1,{x,a}],Hold[Hold[3,{2,a}]]}
> Why is it that in Composition[Hold], Composition does not effectively
> block the HoldAll attribute of Hold? The same thing happens with
> Identity.

It's because Mathematica evaluates the head first and then the body. In 
the first case the head evaluates to Hold:

In[3]:= Composition[Hold]
Out[3]= Hold


Composition[Hold][x+1,{x,a}] ---> Hold[x+1,{x,a}]

and Mathematica doesn't evaluate the body because of the Hold. In the 
second case the head doesn't evaluate to anything:

In[4]:= Composition[Hold, Hold]
Out[4]= Composition[Hold,Hold]

So, the body gets evaluated, and then the Composition takes place

Composition[Hold,Hold][x+1,{x,a}] --->
Composition[Hold,Hold][3,{2,a}] --->

This can be seen by looking at the Trace.

Carl Woll
Wolfram Research

> On 7/31/06, Carl K. Woll <carlw at> wrote:
>> Chris Chiasson wrote:
>> > The above command crashes my kernel on Windows XP and Linux. Anybody 
>> else?
>> >
>> The crash comes when evaluating
>> FindMinimum[3,{2,a}]
>> Presumably you did not intend to evaluate this.
>> In the development version, the above code does not cause a crash.
>> Carl Woll
>> Wolfram Research

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