[zmonx]

In my opinion, this article puts too much emphasis on reading, and too little emphasis on actually reasoning about programs in homoiconic languages like Prolog and Lisp, and due to this imbalance the conclusion is not sufficiently justified.

It is true: Being able to "read without parsing" is definitely nice.

But that is only a subset of those advantages that a homoiconic language gives you. An at least equally important advantage is due to the fact that programs in homoiconic languages are typically very easy to reason about by built-in mechanisms in that language.

For example, Prolog programs are readily represented as Prolog terms, and can be easily reasoned about by built-in mechanisms such as unification.

Since I regard it as a key advantage of homoiconic languages that their abstract syntax is completely uniform and can typically be easily reasoned about within such languages, I disagree with the main point that the article is trying to make.

One interesting fact about homoiconicity is that extremely low-level languages (like assembly code) and extremely high-level languages (like Prolog) are homoiconic, yet there is a large gap "in the middle", where there are many languages (like Java, C, Python etc.) that lack this property.

[icebraining]

Agreed; I've written Python code that processes Python code (both using ast and redbaron), and the result was quite opaque, unlike my metaprograms in Prolog.

In fact, the macropy project[1] offers the "read" step (by abusing the import system), and while using them is pretty cool, I don't think the implementation of the macros is very nice.

[1] https://github.com/lihaoyi/macropy

[catnaroek]

> very easy to reason [sic] about by built-in mechanisms in that language.

Languages allow you to express your reasoning, but they don't do the reasoning by themselves. Also, there is no conclusive evidence that homoiconic languages have simpler semantics, especially of the denotational kind.

[taeric]

I think the meaning is that fewer things are implemented by bringing in outside capabilities. In lisp, everything's can be expressed in terms of other lisp code. Typically directly so. In other languages that are not as macro friendly, most keywords get the explanation of "this is how the computer will act" and then explanations of new behaviors.

[catnaroek]

> In lisp, everything's can be expressed in terms of other lisp code.

Um, and how exactly are primitive forms defined?

> most keywords get the explanation of "this is how the computer will act" and then explanations of new behaviors.

Have you heard of Hoare logic? The meaning of ordinary ALGOL-style imperative programs can be given in terms of relating preconditions to postconditions. Suppose that you have the Hoare triples:

    {P} foo {Q}
    {Q} bar {R}

Then you can derive the Hoare triple:

    {P} foo; bar {R}

Note that `Q` is not mentioned at all. Hence, any implementation is free to translate the program

    foo; bar

into something that doesn't have `Q` as an intermediate state.

[taeric]

You seem to be arguing past me. If you are just upset that I said everything instead of most things, yeah, obviously some things are defined elsewhere. To that end, how things are actually implemented takes a trip to assembly. (I mainly blame that I'm writing in my phone. Often while in the bus.)

My point was that you don't typically see c constructs explained in terms of other c constructs. This is quite common in lisp. To see lisp constructs explained in terms of other lisp constructs. In large because there are few constructs.

You showing me that you can explain using other logic is kind of my point. It is awesome that you can do this. I recommend the skill. It is still not showing c or Java or Haskell or whatever in terms of themselves.

Note that I think you actually can do this, in large. It is not typically done, though.

[catnaroek]

> To that end, how things are actually implemented takes a trip to assembly.

Don't confuse “defined” with “implemented”. This is the entire point to having an axiomatic semantics!

> My point was that you don't typically see c constructs explained in terms of other c constructs.

Languages can't be entirely defined in terms of themselves. At some point you need to drop down to something else. If most of Lisp is defined in terms of other Lisp constructs, that is perfectly fine, but, for my purposes, i.e., proving things about programs, there are two mutually exclusive possibilities:

(0) The semantics of Lisp is the semantics of its primitive forms, and derived forms are just Lisp's standard library.

(1) So-called “derived” forms have an abstract semantics of their own right, and their implementation in terms of so-called “primitive” forms is, well, an implementation detail.

So, my answer to “most of Lisp is defined in terms of Lisp itself” is “that's cute, but how mathematically elegant is the part of Lisp that is not defined in terms of itself?”

[taeric]

I said explained. Not defined. Not implemented. Definitely not proved. Just explained. There is typically exposition, as well.

So, by all means, keep arguing points I'm not making. I was offering what I suspect the parent post meant by it being easier to reason using the mechanics of the language. Nothing more.

[fao_]

> One interesting fact about homoiconicity is that extremely low-level languages (like assembly code) and extremely high-level languages (like Prolog) are homoiconic, yet there is a large gap "in the middle", where there are many languages (like Java, C, Python etc.) that lack this property.

There are languages like picolisp and guile that try and fill that gap a little :)

[ASipos]

I'd like to know more examples of this

[fao_]

https://picolisp.com/wiki/?home

http://rosettacode.org/wiki/Category:PicoLisp

http://picolisp.com/wiki/?PilOS

[divs1210]

Have you not heard of Clojure?

https://clojure.org

[kkylin]

Julia is homoiconic but has much more complex syntax than Lisp. I personally find Julia macros harder to think about than Lisp or Scheme macros, partly for that reason.

[zmonx]

Please note that the creators of Julia no longer call it homoiconic [1].

The fact that you can access the AST in a language is not sufficient to make it homoiconic. There are several programming languages like Julia that let you access the AST yet are not (conventionally) considered homoiconic.

[1] https://stackoverflow.com/questions/31733766/in-what-sense-a...

[kkylin]

Oh, I didn't realize that. Thanks!