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Re: Dynamic scoping

  • To: mathgroup at smc.vnet.net
  • Subject: [mg129700] Re: Dynamic scoping
  • From: Richard Fateman <fateman at cs.berkeley.edu>
  • Date: Tue, 5 Feb 2013 03:06:10 -0500 (EST)
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On 2/2/2013 11:49 PM, Rolf.Mertig at gmail.com wrote:
...

>
> Block[{Plus = "Plus"}, t = ToString[Plus[1, 2]];
>   Print[t];]
> will do what you want
>

No, I think what he wanted was to display a bug in Mathematica.
You just wrote another program.

To illuminate the bug I tried this:

Addition[x_,y]:=x+y;

Block[{Plus=Addition},Print[Plus[1,2]]]

which returns an infinite stream of errors that look like
Join::heads: Heads PacletManager`Package`LDCfindForDocResource and 
PacletManager`Package`MCfindForDocResource at positions 1 and 2 are 
expected to be the same.

I think the explanation needs some explanation, but setting that aside
I tried these:

Block[{P = Addition}, Print[P[1, 2]]]

prints 3

Block[{P = foo}, Print[P[1, 2]]]

prints foo[1,2]

An explanation may be simply that

Block[{Plus}} ....]

binds the name Plus not to some "undefined object" but to the value of 
Plus in the dynamically enclosing scope.  Indeed, this works as
expected:

Block[{Plus = hello}, Block[{Plus = Plus}, Plus[1, 2]]]

returns hello[1,2]

this explanation is not accurate because

Block[{Plus = hello}, Block[{Plus}, Plus[1, 2]]]

returns 3.


So Block[{Plus}, ...
binds the name Plus, or any other Global symbol? to the Global value.

I think that this is a bug.

RJF










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