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C3: Block comments and mega comments.

Christoffer Lernö
1 month, 1 week ago
For C3 I wanted to address the problem of commenting out code using block comments.

In a good overview, Dennie Van Tassel outlined four different types of comments:

  1. Full line comments, this is exemplified by REM in BASIC: The line only contains the comment and it runs to the end of the line.
  2. End-of-Line comments, in C/C++ that would be //
  3. Block comments, '/* ... */' in C/C++
  4. Mega comments, which in C/C++ can be emulated by using // on every line or using the preprocessor with #if 0 ... #endif


We can ignore the full line comments, they're completely covered by end-of-line comments, and C3 already has those and /* ... */ block comments.

However, the mega comments poses a problem. In C3 the analogue to #if 0 ... #endif is $if 0 ... $endif, but it would require the code inside to parse.

Since a typical case for using mega comments would actually be to copy a slab of C code inside of comments and then convert it piecemeal $if 0 doesn't work.

What about making /* ... */ nesting?

In an article from 2017 titled Block Comments are a Bad Idea Troels Henriksen argues that adding nesting to block comments does not really solve the problem and shows the following example from Haskell which uses {- ... -} for nested comments:

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{-
s = "{-";
-}


In the above example the {- inside of the string inadvertently opens a new nested comment. He rejects the idea that the lexer (or even worse, *the parser*) should track strings inside of comments. Instead Henriksen argues for either using #if 0 or // on every line. While the latter is exactly what Zig picked, it relies too much on the text editor for my taste.

Looking at D, it introduces a new nested comment /+ ... +/. It acts just like /* ... */ except it is nested. Initially this was what I picked for C3.

However it has drawbacks:

  1. It introduces another comment type that is only marginally different from the others.
  2. It can have the s = "/+" problem just like "/*" – we just moved the problem.
  3. For beginners coming from C it's not obvious that this comment type is available, so it may get under used.
  4. It does not visually indicate that it should be used for mega comments rather than regular comments.


There's another point as well: #if 0 ... #endif can never have the s = "/*" issue by virtue of always starting and ending on its own line.

Doing some research I tried to determine if there was some "obvious" syntax that could convey the #if 0 ... #endif behaviour. I had a lot of examples (that I hated), like /--- ... ---/ /--> ... /<--- and even ideas of a heredoc style comment like /$FOO ... /$$FOO.

Ultimately I decided to pick /# ... #/ for these block comments, which acted like nested comments but were required to be on a new line which bypasses this problem:

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/#
s = "/#"; <- not recognized
#/


But it turns out that this has issues of its own. What if you by accident write something like:

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/#
int x;
int y = foo(); #/


or

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foo() /#
int x;
#/


You need a good heuristic to figure out a nice error message for these. For example you could either always decide that /#foo is /# + foo or maybe it's only like that if the /# starts a line, otherwise it's interpreted as / + #foo (which can be valid C3).

But after playing around with this for a while, I had to say that the value from this seemed much less than I had hoped. Yes, it's distinct, but it has most of the problems with /+ ... +/ in terms of lack of familiarity. And if I'm honest with myself, I'm personally still mostly using /* ... */ over #if 0 ... #endif where I can.

So we've come full circle: nesting /* */ to distinct nesting block comments, to #if 0 ... #endif and now back to perhaps nesting /* ... */?

For now at least, C3 will add nesting to /* ... */ and remove /+ ... +/. This is an imperfect solution, but possibly also a reasonable trade off to keep the language familiar with features that pull their weight.
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