GHC and C++

Recently I was getting error messages from GHC while building Haskell programs that complain about unknown symbols referring to C++ standard template library symbols. It turns out that these are weak symbols (since only one definition is required) and telling ghc to pass the extra (forbidden, deprecated, and evil) -fno-weak flag to gcc helps. I guess I am in trouble when this flag is finally removed.

After changing this, the compiler complained that it could not find libstdc++.so. A ghc bug describes this problem. GHC was apparently not looking at decorated names and did not find libstdc++.so.6. There is a patch in ghc-HEAD (what will become ghc 7.4).

Since these problems only seemed to arise while the compiler was expanding Template Haskell code. I worked around the problem by moving the modules that link against C++ into a separate library that is only linked after the TH code is already expanded. This way the special GHC compile-time/GHCi linker doesn’t need to worry about these difficult cases.