Homoiconicity in Lisp is easy to see:
(+ 1 2)
is both the function call to + with 1, 2 as arguments, as well as being a list containing +, 1, and 2. It is simultaneously both code and data.
In a language like Julia, though:
1 + 2
I know we can parse this into an Expr in Julia:
:(1 + 2)
And then we can get the AST and manipulate it:
julia> Meta.show_sexpr(:(1+2))
(:call, :+, 1, 2)
So, we can manipulate a program's AST in Julia (and Elixir). But are they homoiconic in the same sense as Lisp- is any snippet of code really just a data structure in the language itself?
I don't see how code like 1 + 2 in Julia is, immediately, data- like how (+ 1 2) in Lisp is just a list. Is it still homiconic, then?
In the words of Bill Clinton, "It depends upon what the meaning of the word 'is' is". Well, ok, not really, but it does depend on what the meaning of the word "homoiconic" is. This term is sufficiently controversial that we no longer say that Julia is homoiconic – so you can decide for yourself whether it qualifies. Instead of trying to define homoiconicity, I'll quote what Kent Pitman (who knows a thing or two about Lisp) said in a Slashdot interview back in 2001:
I like Lisp's willingness to represent itself. People often explain this as its ability to represent itself, but I think that's wrong. Most languages are capable of representing themselves, but they simply don't have the will to. Lisp programs are represented by lists and programmers are aware of that. It wouldn't matter if it had been arrays. It does matter that it's program structure that is represented, and not character syntax, but beyond that the choice is pretty arbitrary. It's not important that the representation be the Right® choice. It's just important that it be a common, agreed-upon choice so that there can be a rich community of program-manipulating programs that "do trade" in this common representation.
He doesn't define homoiconicity either – he probably doesn't want to get into a definitional argument any more than I do. But he cuts to the heart of the matter: how willing is a language to represent itself? Lisp is willing in the extreme – you can't even avoid it: the representation of the program as data is just sitting right there, staring you in the face. Julia doesn't use S-expression syntax, so the representation of code as data is less obvious, but it's not hidden very deep:
julia> ex = :(2a + b + 1)
:(2a + b + 1)
julia> dump(ex)
Expr
head: Symbol call
args: Array(Any,(4,))
1: Symbol +
2: Expr
head: Symbol call
args: Array(Any,(3,))
1: Symbol *
2: Int64 2
3: Symbol a
typ: Any
3: Symbol b
4: Int64 1
typ: Any
julia> Meta.show_sexpr(ex)
(:call, :+, (:call, :*, 2, :a), :b, 1)
julia> ex.args[3]
:b
julia> ex.args[3] = :(3b)
:(3b)
julia> ex
:(2a + 3b + 1)
Julia code is represented by the Expr type (and symbols and atoms), and while the correspondence between the surface syntax and the structure is less immediately obvious, it's still there. And more importantly, people know that code is simply data which can be generated and manipulated, so there is a "rich community of program-manipulating programs", as KMP put it.
This is not just a superficial presentation of Julia code as a data structure – this is how Julia represents its code to itself. When you enter an expression in the REPL, it is parsed into Expr objects. Those Expr objects are then passed to eval, which "lowers" them to somewhat more regular Expr objects, which are then passed to type inference, all implemented in Julia. The key point is that the compiler uses the exact the same representation of code that you see. The situation is not that different in Lisp. When you look at Lisp code, you don't actually see list objects – those only exist in the computer's memory. What you see is a textual representation of list literals, which the Lisp interpreter parses and turns into list objects which it then evals, just like Julia. Julia's syntax can be seen as a textual representation for Expr literals – the Expr just happens to be a somewhat less general data structure than a list.
I don't know the details, but I suspect that Elixir is similar – maybe José will chime in.
See also:
来源：https://stackoverflow.com/questions/31733766/in-what-sense-are-languages-like-elixir-and-julia-homoiconic