Are preprocessors obsolete in modern languages?

Question

Hello, I'm making a simple compiler for a simple pet language I'm creating and coming from a C background(though I'm writing it in Ruby) I wondered if a preprocessor is necessary.

What do you think? Is a "dumb" preprocessor still necessary in modern languages? Would C#'s conditional compilation capabilities be considered a "preprocessor"? Does every modern language that doesn't include a preprocessor have the utilities necessary to properly replace it? (for instance, the C++ preprocessor is now mostly obsolete(though still depended upon) because of templates.)

Answer 1

The preprocessor is a cheap method to provide incomplete metaprogramming facilities to a language in an ugly fashion.

Prefer true metaprogramming or Lisp-style macros instead.

Answer 2

I think that preprocessors are a crutch to keep a language with poor expressive power walking.

I have seen so much abuse of preprocessors that I hate them with a passion.

Answer 3

C's preprocessing can do really neat things, but if you look at the things it's used for you realize it's often for just adding another level of abstraction.

• Preprocessing for different operations on different platforms? It's basically a layer of abstraction for platform independence.
• Preprocessing for easily adding complex code? Abstraction because the language isn't generic enough.
• Preprocessing for adding extensions into your code? Abstraction because your code / your language isn't flexible enough.

So my answer is: you don't need a preprocessor if your language is high-level enough *. I wouldn't call preprocessing evil or useless, I just say that the more abstract the language gets, the less reason I can think for it needing preprocessing.

* What's high-level enough? That is, of course, entirely subjective.

EDIT: Of course, I'm only really referring to macros. Using preprocessors for interfacing with other code files or for defining constants is evil.

Answer 4

A preprocessor is a separated phase of compilation. While preprocessing can be useful in some cases, the headaches and bugs it can cause make it a problem.

In C, preprocessor is used mostly for:

1. Including data - While powerful, the most common use-cases do not need such power, and "import"/"using" stuff(like in Java/C#) is much cleaner to use, and few people need the remaining cases;
2. Defining constants - Why not just provide a "const" statement
3. Macros - While C-style macros are very powerful(they can include statements such as returns), they also harm readability. Generics/Templates are cleaner and, while less powerful in a few ways, they are easier to understand.
4. Conditional compilation - This is possibly the most legitimate use-case for preprocessors, but once again it's painful for readability. Separating platform-specific code in platform-specific source code and using common if statements ends up being better for readability.

So my answer is while powerful, the preprocessor harms readability and/or isn't the best way to deal with some problems. Newer languages tend to consider code maintenance very important, and for those reasons the preprocessor appears to be obsolete.

Answer 5

It's your language so you can build whatever capabilities you want into the language itself, without a need for a preprocessor. I don't think a preprocessor should be necessary, and it adds a layer of complexity and obscurity on top of a language. Most modern languages don't have preprocessors, and in C++ you only use it when you have no other choice.

By the way, I believe D handles conditional compilation without a preprocessor.

Answer 6

It depends on exactly what other features you offer. For example, if I have a const int N, do you offer for me to take N variables? Have N member variables, take an argument to construct all of them? Create N functions? Perform N operations that don't necessarily work in loops (for example, pass N arguments)? N template arguments? Conditional compilation? Constants that aren't integral?

The C preprocessor is so absurdly powerful in the proper hands, you'd need to make a seriously powerful language not to warrant one.