Why are there no languages that are both interpreted and (really) compiled?

Question

I am an (old) engineer not a programmer so forgive me for asking a naïve question.

My understanding is that to get really fast execution times for a program, it needs to be compiled to native machine code. And there are a relatively small number of languages still in use that do this (e.g. C and C++).

But I much prefer the syntax of Python over that of the C-derived compiled languages. However my understanding is that interpreted Python (and pseudo-compiled Python run on a virtual machine) cannot match the execution speed of a truly compiled language.

Is there some reason that a true native-code Python compiler cannot be developed?

[I am interested specifically in Python but I am not aware of any language that can be interpreted and also compiled to native machine code.]

Answer 1

The key distinction is a clean separation between compile time and run time. In Python, for instance, import happens at runtime, and can happen conditionally. And per the halting problem, that means a compiler cannot determine up front if a given import will happen. Yet, this affects the code that would need to be generated.

As Bill the Lizard notes, if the language does have a clean distinction, an interpreter can still choose to ignore it. C's #include can happen before main, but that does not mean an interpreter must do so.

Outside the syntax, Python is also virtually uncompilable due to weak typing. In C, + has a very limited set of meanings - integer, floating point or pointer, and the compiler will know the static type of the arguments. C++ has far more extensive overloading, but the same basic principle applies. virtual functions allow some run-time flexibility, but from an finite set of options, all compiled before main starts.

Also not syntax is the memory model - both C and C++ have a memory model that's an improved derivative of Java's memory model, which makes threading quite efficient. (Unlike Java, not every object can be synchronized, you need special members). As CPU's gain more and more cores, the advantages only continue to grow. Compilers can see pretty well where memory and CPU registers need to be brought in sync.