Return-Path: <kfoury@cs.bu.edu> X-Spam-HitLevel: X-Spam-DCC: sonic.net: cs3.bu.edu 1156; Body=1 Fuz1=1 Fuz2=1 X-Spam-Checker-Version: SpamAssassin 3.1.0 (2005-09-13) on cs3.bu.edu X-Spam-Level: X-Spam-Status: No, score=-4.4 required=10.0 tests=ALL_TRUSTED,AWL,BAYES_00 autolearn=ham version=3.1.0 X-Spam-Pyzor: Received: from [76.19.13.75] ([76.19.13.75]) (authenticated bits=0) by cs3.bu.edu (8.13.6/8.13.6) with ESMTP id k9N4XBmk026128; Mon, 23 Oct 2006 00:33:22 -0400 Message-ID: <453C4618.2010202@cs.bu.edu> Date: Mon, 23 Oct 2006 00:33:28 -0400 From: Assaf Kfoury <kfoury@cs.bu.edu> User-Agent: Mozilla Thunderbird 1.0.7 (X11/20051013) X-Accept-Language: en-us, en To: Michel Machado <michel@digirati.com.br> CC: cs520@cs.bu.edu Subject: Re: Evaluation Context - Derivation References: <00e301c6f632$b1451490$16b6a8c0@dark> In-Reply-To: <00e301c6f632$b1451490$16b6a8c0@dark> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Clamav-Status: No Status: RO Content-Length: 1583 X-UID: 26 X-Keywords:
Let t be the term "(lam x. x + 2) 3". The whole of t is a beta-redex.
So, the corresponding evaluation context E for t is simply [ ],
consisting of a hole and nothing else. (I write E instead of E_cbv or
E_cbn, for brevity.)
Here plug(t,[ ]) = t. The whole of t is a beta-redex such that t -> (3 + 2).
I said in lecture that the approach to evaluation (or operational
semantics) based on evaluation contexts is "more scalable". What I meant
is that if we extend the language by adding new programming features,
then the operational semantics based on evaluation contexts "more easily
scales up" to the extended language. This is not a theorem and there is
no proof for it -- it is just an observation based on experience (mine
and that of many other researchers in this area), but for which I did
not give any examples in lecture.
Scalability, i.e. the ability to extend a property from a smaller
language to a larger language, has nothing to do with efficiency or
"faster evaluation".
Assaf
Michel Machado wrote:
> Hi Assaf,
>
> Could you write the derivation of the term (lam x. x + 2) 3 using
> evaluation contexts?
>
> I don't see what the first Ecvb in plug function should be. I mean,
> how can I complete the following expression in order to eval that term?
>
> plug(t, Ecbv) = plug((lam x. x + 2) 3, ????) = ????
>
> Do you have other links about evaluation contexts?
>
> You said in last class that evaluation contexts are more scalable,
> in what sense is it more scalable? Does it lead to faster evaluation?
>
> [ ]'s
> Michel Machado
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