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by Denzel 3329 days ago
No, this is incorrect. The syntax and the AST must be isomorphic for a language to be homoiconic. It's not enough to expose the compiler/AST as a first-class library.

Wikipedia has a nice entry on this. In short, "homoiconicity is where a program's source code is written as a basic data structure that the programming language knows how to access."

[1]: https://en.wikipedia.org/wiki/Homoiconicity
2 comments
kazinator 3329 days ago
According to that page, the term was introduced by the designer of something called TRAC, who used it to denote the idea that the program is stored in the memory in the same form in which the user enters it, which allows it to be inspected and changed. Nothing about ASTs.
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prestonbriggs 3329 days ago
And TRAC is all about macros...

TRAC is lots of fun. I read about it in Nelson's book, Computer Lib/Dreams, when I was a freshman at Illinois. That spring, my Dad bought an Altair and I wrote a version of Trac for it, in assembly. Had support for bignum arithmetic, in ASCII :-)

Later, when I learned about tail recursion, I was happy to figure out that my implementation was indeed properly tail recursive.

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Denzel 3329 days ago
What you said and what I said are in agreeement, so I'm not sure I follow. "Program stored in memory in the same form in which the user enters it" makes it isomorphic. Which was exactly the point I was trying to make in response to the parent comment.

You cannot simply expose the compiler/AST data structure and call your language homoiconic because the text the user enters and the resulting AST are not isomorphic which is a necessary precondition for homoiconicism.

I may be missing something in what you said though, so please do let me know.

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DonbunEf7 3329 days ago
By "first-class", I really did mean "indistinguishable from the rest of the core of the language." Consider Monte:

    def x :Int := 42 # evaluated statement
    def ast := m`def x :Int := 42` # quasi-quoted Monte fragment
    eval(ast, safeScope) # easy evaluation
Now, it happens that m`` is a library written in Monte itself, but that's unsurprising when you consider how much of the Monte compiler is also self-hosting. Since Monte is a complex and rich language, the homoiconic representation is equally rich:

    def m`def @lhs := @rhs` := ast # pattern-matching!
    [lhs, rhs] # [mpatt`x :Int`, m`42`]
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