Can a compiled language be homoiconic?

Question

By definition the word homoiconic means:

Same representation of code and data

In LISP this means that you could have a quoted list and evaluate it, so (car list) would be the function and (cdr list) the arguments. This can either happen at compile- or at run-time, however it requires an interpreter.

Is it possible that compiled languages without a compile-time interpreter can be homoiconic as well? Or is the concept of homoiconicity limited to interpreters?

Answer 1

'Homoiconic' is kind of a vague construct. 'code is data' is a bit clearer.

Anyway, the first sentence on Wikipedia for Homoiconic is not that bad. It says that the language has to have a source representation using its data structures. If we forget 'strings' as source representation (that's trivial and not that helpful to have a useful concept 'homoiconic'), then Lisp has lists, symbols, numbers, strings etc. which are used to represent the source code. The interface of the EVAL function determines what kind of source representation the language is working on. In this case, Lisp, it is not strings. EVAL expects the usual variety of data structures and the evaluation rules of Lisp determine that a string evaluates to itself (and thus will not be interpreted as a program expression, but just string data). A number also evaluates to itself. A list (sin 3.0) is a list of a symbol and a number. The evaluation rules say that this list with a symbol denoting a function as the first object will be evaluated as a function application. There are a few evaluation rules like this for data, special operators, macro applications and function applications. That's it.

To make it clear: in Lisp the function EVAL is defined over Lisp data structures. It expects a data structure, evaluates it according to its evaluation rules and returns a result - again using its data structures.

This matches the definition of homoiconic: source code has a native representation using the data types of Lisp.

Now, the interesting part is this: it does not matter how EVAL is implemented. All that matters is that it accepts the source code using the Lisp data structures, that it executes the code and that it returns a result.

So it is perfectly legal that EVAL uses a compiler.

(EVAL code)  =  (run (compile-expression code))

That's how several Lisp system work, some don't even have an Interpreter.

So, 'Homoiconic' says that the SOURCE code has a data representation. It does NOT say that at runtime this source code has to be interpreted or that the execution is based on this source code.

If the code is compiled, neither the compiler nor an interpreter is needed at runtime. Those would only be needed if the program wants to eval or compile code at runtime - something that is often not needed.

Lisp also provides a primitive function READ, which translates an external representation (S-Expressions) of data into an internal representation of data (Lisp data). Thus it also can be used to translate an external representation of source code into an internal representation of source code. Lisp does not use a special parser for source code - since code is data, there is only READ.

Answer 2

yes. lisp can be compiled to a native binary

Answer 3

Seems to me to be an odd question:

Firstly, the homoiconic portion is the presented interface to the programmer. The point of languages is that they abstract a lower level functionality that preserves the same semantics as the higher level presentation (though a different means).

dsm's machine-code point is a good point, but providing:

1. The syntax and semantics presented are homoiconic
2. The translation to a lower level form (machine code or interpreted or otherwise) doesn't remove any of the original semantics then

why does the lower level implementation matter here?

Also:

compiled languages without a compile-time interpreter

Without some program interpreting it, it would be required to be native to the CPU, therefore the CPU's native language would be required to be homoiconic (or the VM running the code).

Languages without compile-time interpretation ... would be fairly constrained ... as they wouldn't be compiled at all.

But I am no expert, and maybe missing the point.

Answer 4

In the most literal form, C is homoiconic. You can get access to the representation of a function using &functionName and execute data using somePtrCastToFnPtr(SomeArgs). However this is at the machine code level and without some kind of library support you will find it very hard to work with. Some kind of embeddable compiler (I seem to remember that LLVM can do this) would make it more practical.

Answer 5

Lisp is normally compiled. There have been implementations with JIT compilers instead of interpreters.

Hence, it is not necessary to have an interpreter (in the sense of "not a compiler") for code-is-data languages.

Answer 6

Machine code itself is homoiconic, so yes.

Data or instructions are just a matter of semantics (and perhaps the segment of memory in which them lie).

Answer 7

The problem is that a lot of processors separate instruction and data areas, and actively prevent programs from modifying their own code. This kind of code used to be called "degenerate code", and considered a very Bad Thing.

Interpreters (and VMs) don't have that problem, as they can treat the whole program as data, with the only "code" being the interpreter.

Answer 8

Yes; you just have to stick a copy of the compiler into the language runtime. Chez Scheme is one of the many fine compilers which do just that.

Answer 9

Compiling is just optimized interpretation. An interpreter takes a piece of data representing code and then "does" that code: the code's meaning turns into pathways of execution and data flow through the guts of the interpreter. A compiler takes the same data, translates it into another form and then passes it to another interpreter: one implemented in silicon (CPU) or perhaps a fake one (virtual machine).

This why some Lisp implementations are able not to have intepreters. The EVAL function can compile the code and then branch to it. EVAL and COMPILE do not have to have distinct modes of operation. (Clozure, Corman Lisp, SBCL are examples of "compiler only" Lisps.)

The data part in the beginning is the key to the language being homoiconic, not whether or not the execution of code is optimized by compiling. "Code is data" means "source code is data" not "executable code is data". (Of course executable code is data, but by data we mean the overwhelmingly preferred representation of the code that we want to manipulate.)

Answer 10

Languages built on top of VM's (.net clr, jre ect) can use advanced technics that allow on the fly code generation. One of them is IL weaving. Although, its not as clear as eval's of ECMAScript/Lisp/Scheme ect but it can to a some degree emulate such behavior.

For examples check Castle DynamicProxy and for more interactive example check LinqPAD, F# Interactive, Scala interactive.