Another CPP gotcha for the manual

[Mark Lentczner]

Turns out that c pre-processing removes C style (/*…*/) comments. Of
course, it will do this anywhere in the source. The following program can
tell if it is compiled *-cpp* or not:

module Main where

(*/**) :: Double -> Double -> Double
(*/**) a b = a / (2 ** b)

howCompiled = if use > 1/16 then "normal" else "-cpp"
  where
    use = 1 */** 2 + 3 */** 4

main = putStrLn $ "compiled " ++ howCompiled


When run:

& runhaskell CppTest.hs
compiled normal

& runhaskell -cpp CppTest.hs
compiled -cpp


An example in the wild is in the package *wai-extra*, in the file
*Network/Wai/Middleware/RequestLogger.hs* where the */* construct appears
twice in the comments.

Short of defining and implementing our own CPP-like preprocessing
(something we might actually consider), I don't think there really is any
fix for this, so the bug is that it should appear in the GHC documentation
on CPP mode  (§4.12.3), along with similar warnings about trailing
back-slashes.

Note that the way in which a multi-line comment is removed differs between
*gcc* and *clang*. In *gcc*, the comment is removed and content of the line
before the comment, and contents of the line after the comment are joined
into a single line. In *clang*, the two line fragments are kept on separate
lines. In both cases extra empty lines are added to keep the line count the
same.

The consequence of the *gcc* / *clang* difference is that neither the above
code, nor wai-extra will compile with *clang*.

Note: As far as I can tell this is not a *clang* bug, but a failure of
specs: The C definition of comments and their removal is vague, and my
guess is *gcc* choose its method based on historical use. The C++
definition makes it clear that comments are whitespace, even once removed,
and so the *clang* method is correct.

- Mark
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