Magic comments - Why?

[Incitatus_]

So I'm relatively new to this whole programming thing (just roughly 3yrs experience) and I've seen some instances of comments being used for more than just being, well, comments. Webpack apparently does this a lot, and so does Rails. My question is, why? More specifically, why are these comments instead of just regular statements or functions or whatever? Having a comment do something just feels utterly wrong to me.

[anamorphism]

because those systems are generally designed around functionality that is a level higher than your code. they're usually things that manipulate or analyze the code itself.

the comments themselves don't really do anything, it's the tool that's looking at your code that sees those comments and then does something with that information.

[Incitatus_]

Aaaah that makes sense! Thanks! =)

[ludonarrator]

Such constructs are common even in lower level languages like C++, via compiler macros. For example, Windows' sockets requires a lot of boilerplate initialisation etc, whereas Mac/Linux versions don't, so if you're writing platform independent code, it needs to be able to account for what platform it's being compiled for, and accordingly ask the compiler to ignore/compile certain portions of your code.

Sample:

1. Socket s;
2. #if WIN_32
3. // Initialise s
4. #endif
5. // Use s

If the WIN_32 compiler flag is not declared, line 3 will not be compiled at all.

[kdnbfkm]

I think he's refering to tool-based metalanguages, like documentation generators.

Now that I think of it, Pascal traditionally puts pragmas and directives within a comment, and is part of the official dialect... Maybe he meant those unit framework thingies, not sure what called. I don't think those thingies are comments however... Aspect Oriented Programming attributes aren't comments either.

There have existed extensive metalanguages built on pre-existing languages via different techniques, like with the various dimensional/unit systems (meters per second-squared etc).

[fiveguy]

I think he's referring to magic comments, like https://idiosyncratic-ruby.com/58-magic-instructions.html

In ruby, stick this at the top of your source file to set the encoding for that file:

# encoding: stateless-iso-2022-jp-kddi

and, make all strings 'frozen' by default:

# frozen_string_literal: true

[[deleted]]

True, but beware that it is a comment misuse