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    <blockquote> <p>In the first version <code>(+ 1 2 3)</code> is raw code, whereas in the second version it is data. By assuming the truth of this statement it can be argued that Lisp isn't even homiconic. The code has the same representation as data in the sense that they are both lists/trees/S-expressions. But the fact that you have to explicitly mark which of these lists/trees/S-expressions are code and which are data to me seems to say that Lisp is not homiconic after all.</p> </blockquote> <p>This is not true. In the first version, the list <code>(+ 1 2 3)</code>, which is data, is being fed to the interpreter to be executed, i.e. to be <em>interpreted as code</em>. The fact that you have to mark s-expressions as being code or data <em>in a specific context</em> does not make Lisp non-homoiconic.</p> <p>The point of homoiconicity is that all programs are data, not that all data are programs, so there is still a difference between the two. In Lisp, <code>(1 2 3)</code> is a valid list but not a valid program since an integer is not a function.</p> <p>[If we look at the other great homoiconic programming language, Prolog, then we see the same phenomenon: we can build a data structure <code>foo(X, 1, bar)</code>, but without a definition of <code>foo</code>, we can't execute it. Also, variables cannot be the names of predicates or facts, so <code>X.</code> is never a valid program.]</p> <p>Lisp is self-modifying to a great degree. E.g., here's how to change the definition of a function:</p> <pre><code>[1]&gt; (defun foo (x) (+ x 1)) FOO [2]&gt; (defun bar (x) (+ x 2)) BAR [3]&gt; (setf (symbol-function 'foo) #'bar) #&lt;FUNCTION BAR (X) (DECLARE (SYSTEM::IN-DEFUN BAR)) (BLOCK BAR (+ X 2))&gt; [4]&gt; (foo 3) 5 </code></pre> <p>Explanation: at <code>[1]</code>, we defined the function <code>foo</code> to be the add-1 function. At <code>[2]</code>, we defined <code>bar</code> to be the add-2 function. At <code>[3]</code>, we reset <code>foo</code> to the add-2 function. At <code>[4]</code>, we see that we've successfully modified <code>foo</code>.</p>
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    1. CO"The point of homoiconicity is that all programs are data, not that all data are programs" Hadn't thought of that before. So what's the term that I'm looking for? In machine code it's homoiconic in both directions. code->data and data->code. I'm looking for languages of that sort. Cheers for shooting down my argument with a solid refutation btw :)
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    2. COSome (many? all?) Lisps also provide an s-expression interface to the assembly-language / machine-code / bytecode form of a compiled expression (e.g. `(disassemble #'bar)`), but I've never been (brave? desperate?) enough to attempt altering it in that form. I'd argue that, even without that function, the homoiconicity is in relation to the AST, though, and not the machine-dependent, compiled form.
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    3. COLisp is "as" homoiconic as any machine code, in that all code is data, but not all data is *valid* code. You can try to execute `$f00fc7c8` as an instruction, but there's not guarantee that the processor can make sense of it.
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