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POWhy doesn't Scheme support first class environments?
 

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    <p>I've been reading through SICP (Structure and Interpration of Computer Programs) and was really excited to discover this wonderful special form: "make-environment", which they demonstrate to use in combination with eval as a way of writing modular code (excerpt from section 4.3 on "packages"):</p> <pre><code>(define scientific-library (make-environment ... (define (square-root x) ...))) </code></pre> <p>They then demonstrate how it works with</p> <pre><code>((eval 'square-root scientific-library) 4) </code></pre> <p>In their example, they then go on to demonstrate <em>exactly</em> the usage that I would want - an elegant, minimalist way of doing the "OO" style in scheme... They "cons" together a "type", which is actually what was returned by the "make-environment" special form (i.e. the vtable), and an arg ("the state")... </p> <p>I was so excited because this is <em>exactly</em> what I've been looking for as a way to do polymorphic dispatch "by symbol" in Scheme without having to write lots of explicit code or macros.</p> <p>i.e. I want to create an "object" that has, say, <em>two</em> functions, that I call in different contexts... but I don't want to refer to them by "car" and "cdr", I want to both <em>declare</em> and <em>evaluate</em> them by their symbolic names.</p> <p>Anyway, when I read this I couldn't wait to get home and try it.</p> <p>Imagine my disappointment then when I experienced the following in both PLT Scheme and Chez Scheme:</p> <pre><code>&gt; (make-environment (define x 3)) Error: invalid context for definition (define x 3). &gt; (make-environment) Error: variable make-environment is not bound. </code></pre> <p>What happened to "make-environment" as referenced in SICP? It all seemed so elegant, and exactly what I want, yet it doesn't seem to be supported in any modern Scheme interpreters?</p> <p>What's the rationale? Is it simply that "make-environment" has a different name?</p> <p><strong>More information found later</strong></p> <p>I took at look at the online version:</p> <p><a href="http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-28.html#%_sec_4.3" rel="nofollow noreferrer">http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-28.html#%_sec_4.3</a></p> <p>I was reading was the first edition of SICP. The second edition appears to have replaced the discussion on packages with a section on non-deterministic programming and the "amp" operator.</p>
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    1. USPaul Hollingsworth
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