Can the equivalent of eval and exec exist in a compiled language? If so, how would they be compiled (roughly speaking)?
2 Answers
For starters, python is a compiled language, it just does the compilation at runtime. That being said, all that you need to do to implement eval in any other compiled language is to be able to run the compiler (and dynamically load object code) - you can do this in Python (and a litany of other languages) easily because the compiler is an integral part of the runtime. There's technically nothing that stops a program written in C from invoking the compiler and loading the result at runtime (using dlopen), it's just not a common occurrence because the C runtime doesn't require a compiler, so most users don't have one.
2 Comments
slebetman
Not just that but C is also a language that is known to be inherently hard to compile so invoking the compiler to "eval" something may potentially force the user to wait 5 minutes or worse. Python cheats a bit because it doesn't compile to machine code but to a virtual machine so you can make the virtual machine support complex stuff to make the language easier to compile. Lisp is easy to compile to begin with because the source code is basically already an abstract syntax tree. Another easy to compile language is Forth - the original editor compiled each line of code as it is typed.
Nick Bastin
@slebetman: actually there are systems that invoke the C compiler, and they're very fast (some modes of pypy are one example) - modern C compilers are very very fast (not C++), and in fact the most time consuming component of the build cycle in C is often the linker (depending on the platform), and if you can shorten/eliminate that, the size of the code fragments you're usually
eval ing usually don't cause significant delays (as a point of reference, icl compiles lines of C to machine code faster than the python compiler converts lines of python to VM opcodes).Certainly, various Lisp environments have had this capability for decades. A Lisp compiler typically works on a per-function basis, and the compiler and runtime system work hand in hand.
When asked to eval something, the Lisp runtime environment will pass the list (a data structure) to the compiler for compiling. The compiler may generate machine code (or maybe bytecode, depending on the system), and then the function will be callable at the machine level just like every other function in the program.
