I've built an interpreter in C++ and everything works fine so far, but now I'm getting stuck with the design of the import/include/however you want to call it function.

I thought about the following:

• Handling includes in the tokenizing process: When there is an include found in the code, the tokenizing function is recursively called with the filename specified. The tokenized code of the included file is then added to the prior position of the include. Disadvantages: No conditional includes(!)

• Handling includes during the interpreting process: I don't know how. All I know is that PHP must do it this way as conditional includes are possible.

Now my questions:

• What should I do about includes?
• How do modern interpreters (Python/Ruby) handle this? Do they allow conditional includes?