CPP extension and multiline literals in Haskell

Question

Is it possible to use CPP extension on Haskell code which contains multiline string literals? Are there other conditional compilation techniques for Haskell?

For example, let's take this code:

-- If the next line is uncommented, the program does not compile.
-- {-# LANGUAGE CPP #-}

msg = "Hello\
  \ Wor\
  \ld!"

main = putStrLn msg

If I uncomment {-# LANGUAGE CPP #-}, then GHC refutes this code with a lexical error:

[1 of 1] Compiling Main             ( cpp-multiline.hs, cpp-multiline.o )

cpp-multiline.hs:4:17:
    lexical error in string/character literal at character 'o'

Using GHC 6.12.1, cpphs is available.

I confirm that using cpphs.compat wrapper and -pgmP cpphs.compat option helps, but I'd like to have a solution which does not depend on custom shell scripts. -pgmP cpphs does not work.

P.S. I need to use different code for GHC < 6.12 and GHC >= 6.12, is it possible without preprocessor?

UPD. In addition to the accepted answer of Ganesh, I also found that another workaround is to put all conditional declarations in a separate module with {-# LANGUAGE CPP #-} and thus avoid CPP in the modules with multiline strings.

Monochrome

Why not use "Hello" ++ " Wor" ++ "ld!"? – Lunatic

Because "Hello\ whitespace+ \ World!" is Haskell 98. Because there is some code around which uses it. Because " ++ " takes more space (and I dislike code beyond 80 columns). And finally, I'd like to write a patch without re-wrapping and re-formatting every multiline string in the project. – Monochrome

Answer 1

cpphs now has a --cpp option itself, which I think makes the compat script unnecessary: see the cpphs 1.3 entry at http://haskell.org/cpphs/

I think you'd need to pass -optP --cpp to GHC (as well as -pgmP cpphs) to enable this behaviour.

Bunting

Answer 2

It seems the GHC user manual addresses this: Section 4.10.3.1 reads

A small word of warning: -cpp is not friendly to “string gaps”.. In other words, strings such as the following:

strmod = "\
\ p \
\ "

don't work with -cpp; /usr/bin/cpp elides the backslash-newline pairs.

However, it appears that if you add a space at the end of the line, then cpp (at least GNU cpp and possibly other cpps) leaves the backslash-space pairs alone and the string gap works as expected.

Exerciser

Thank you. I tried to add trailing spaces after \, but cpp just reports a warning backslash and newline separated by space, and the code still does not compile. Using GNU cpp 4.4.3. – Monochrome

Answer 3

The decision is

strmod = "\
 p \
 "

No spaces after backslash needed plus no backslash needed at the line begin.

Sock