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    <p>Lisp is a family of programming languages. Members of this family are widely different in their capabilities and implementation techniques. There are two different interpretations of this:</p> <ul> <li><p>Lisp is a family of languages which share some overlapping feature set. This includes everything from the first Lisp, to Maclisp, Scheme, Logo, Common Lisp, Clojure and hundreds of different languages and their implementations.</p></li> <li><p>Lisp also has a main branch of languages which also mostly have 'Lisp' in their name: Lisp, MacLisp, Common Lisp, Emacs Lisp, ...</p></li> </ul> <p>Lots of research has been invested over time to improve the languages (make it more 'Functional' or make it more object-oriented, or both) and to improve implementation techniques.</p> <p>Common Lisp for example supports compilation, various optimizations, and more to allow developers to use it in large projects where some balance between flexibility and features is needed. A compiled function is then machine code and no longer a data structure made of lists, symbols, strings and numbers.</p> <p>Common Lisp allows implementations to create static code. At the same time it leaves some place for controlled runtime modifications (for example by using a runtime compiler, loading code, evaluating code, replacing code, ...).</p> <p>OTOH if you have a Lisp implementation with an interpreter and additionally the interpreter might use Lisp data structures to represent source, then you are able to change running programs for example by changing the list structure. There are implementations of dialects of Lisp which allow that. One typical thing is the use of macros, which can be computed at runtime by such a system. Some other Lisps have so-called Fexprs, which are a similar mechanism (but which usually can't be effectively compiled).</p> <p>In a Common Lisp based application (say, a CAD system written in Lisp) where much of the source information has been removed by a delivery tool, this would not be possible. One would have a single executable of machine code, which has much of the runtime flexibility removed.</p> <p>Homoiconicity is also not a very well defined concept. For Lisp I like more that we say that source code can be turned into data, because the source code uses the external representation of s-expressions, which is a data serialization format. But not every s-expression is a valid Lisp program. Also an internal representation of source code as Lisp data is not 'iconic' in any way. Lisp has source code as external s-expressions and after reading the source code, it has an internal representation as Lisp data. READ reads it, PRINT prints it and EVAL evaluates it.</p> <p>There are also other approaches in Lisp to provide access to the Lisp which is running your program and to the program:</p> <ul> <li><p>the meta-object protocol in CLOS (the Common Lisp Object System) is such an example</p></li> <li><p>3Lisp provides an infinite tower of interpreters. There is a Lisp interpreter running your program. This Lisp interpreter is ran by another Lisp interpreter, which is again running in another one, and that one too...</p></li> </ul>
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    1. POHomoiconic and "unrestricted" self modifying code + Is lisp really self modifying?
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    1. USRainer Joswig
    1. USRainer Joswig
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    1. COSeconding **3Lisp**. If you ever wanted to see reflective programming pushed to the extreme, 3Lisp will open your eyes.
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