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MathGroup Archive 2001

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Re: scope all wrong? in Mathematica 4.1

  • To: mathgroup at smc.vnet.net
  • Subject: [mg31847] Re: scope all wrong? in Mathematica 4.1
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Fri, 7 Dec 2001 05:56:38 -0500 (EST)
  • References: <9ul2ft$6m3$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Richard,

Richard,
I have found a feature amongst your examples that came as a surprise: the
way in which SetDelayed is evaluated.  The outputs from your examples seem
to be consequences of this and the standard evaluation process - I'll come
back to them later {in the notebook below].
My understanding was that  h[e]:=r evaluates h then e to given h1[e1]:=r;
then stores the rule for this (without looking for rules that might change
it as a whole); it then returns the value Null.
It seems however that more is going on:  the rule is stored is as expected
from the above, but it seems that after making later rules; some kind of
evaluation is triggered, as though trying to evaluate the left sides using
the other rules (the rules themselves are not altered). These evaluations
may be due to working out the order in which to store or replace existing
ones.  It seems to me that it would be better if these evaluations were done
in private. However, the same kind of evaluations will occur when the rules
are used, and any unwanted assignments can be avoided in both circumstances
by suitable localisation of  symbols inside tests and conditions, though
unwanted printing would still occur.

Please see the notebook below for details.

--
Allan

---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565


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Cell["\<\
Richard,
I have found a feature amongst your examples that came as a surprise: \
the way in which SetDelayed is evaluated.  The outputs from your \
examples seem to be consequences of this and the standard evaluation \
process - I'll come back to them later.
My understanding was that  h[e]:=r evaluates h then e to given \
h1[e1]:=r; then stores the rule for this (without looking for rules \
that might change it as a whole); it then returns the value Null.
It seems however that more is going on:  the rule is stored is as \
expected from the above, but it seems that after making later rules; \
some kind of evaluation is triggered, as though trying to evaluate \
the left sides using the other rules (the rules themselves are not \
altered). These evaluations may be due to working out the order in \
which to store or replace existing ones.  It seems to me that it \
would be better if these evaluations were done in private. However, \
the same kind of evaluations will occur when the rules are used, and \
any unwanted assignments can be avoided in both circumstances by \
suitable localisation of  symbols inside tests and conditions, though \
unwanted printing would still occur.
Here are some examples\
\>", "Text"],

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However, the stored rules are still as we expected - unaffected by \
the extra evaluations:\
\>", "Text"],

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I have modified your examples slightly to make some points - please \
allow for these in interpreting the quotes from your posting.\
\>", "Text"],

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This is nothing to do with Block: it occurs because passing (5) \
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This  is because inside the inner Block x is temporarily set the \
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\>", "Text"],

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>Using pattern matching for
>substituting 4 for x in rr[4] even when x is bound seems
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Function: Block does not bind - as remarked earlier it effect is via \
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>Now look at\
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As you see, I did not get anything.\
\>", "Text"],

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>x is 5 printed
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>Try this\
\>", "Text"],

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I also reset x to 100 and change your 5 to 6 , resmake a point about \
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As we expect from the first section, printouts pile up even if we \
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But with no such rule another feature emerges:
Check:\
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(*******************************************************************
End of Mathematica Notebook file.
*******************************************************************)




"Richard Fateman" <fateman at cs.berkeley.edu> wrote in message
news:9ul2ft$6m3$1 at smc.vnet.net...
> Consider the mathematica definition
>
> rr[x_] := Block[{x}, x = 5; Print["x is ", x]]
>
>   what do you expect here?
>
> rr[z]  -->   x is 5  is printed.
> but you might not expect
>
> rr[4]  -->    "Block::"lvsym": "Local variable specification{4} contains 4
> which is not a symbol or an assignment to a symbol."
>
> If you use Module instead of Block, you get
> the same.
>
> However,
> Block[{x = 4}, Block[{x}, x = 5; Print["x is ", x]]]
>
> prints   x is 5
>
> Using pattern matching for
> substituting 4 for x in rr[4] even when x is bound seems
> to be counter to what most programming language designers
> would expect.
>
>
>
> Now look at
>
> uu[x_?((x = 5; Print["x is ", x]; True) &)] := x
>
> Usually when one defines a program, nothing is printed.
> Here, we get
> x is 5
> 5
> x is 5
>
> If we do this:
> x =400
> uu[70]
>      x is 5  printed
> the value returned is 70
> and the global x is 5.
>
> Try this:
> Module[{x}, uu[x_?((x = 5; Print["x is ", x]; True) &)] := x;
>    Print["in Module, x is " , x]]
>
> The expectation in a system supporting lexical scope
> with Module
> is that no use of x inside the module would escape.
> But it does.  The global x is set to 5.
>
>
> The reason this all came up is in correspondence suggesting
> that programs in one computer algebra system could be
> translated into another.  If systems are semantically
> "surprising", it is more difficult. I wonder how much
> of mathematica internally depends on wrong scope, or how
> much of the code is susceptible to bugs because of unexpected
> capture of names.  (I suspect that this has caused a proliferation
> of package names e.g. MySecretNameSpace`x   in Mathematica
> routines).
>
> RJF
>
>




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