The other neat thing about modular and functional programming is that since modules/functions are discrete, you can drop in any replacement you want to, without

The other neat thing about modular and functional programming is that since modules/functions are discrete, you can drop in any replacement you want to, without the rest of the codebase even noticing (except during the build process). As a real example, you can use Haskell's Foreign Function Interface (FFI) to call C functions, and wrap C structs. The rest of your Haskell code doesn't even need to know about it. You can use the same interface with a variety of languages in place of C. Rust even has a specific compiler option for generating C-compatible shared object/dll files.