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GHCi fails to load modules with weak symbols. The compiler, in contrast, has no trouble with them. The attached Cabal package demonstrates the problem. After building and installing:

:Prelude> :m +WeakTest
Prelude WeakTest> weak_test 0
Loading package WeakTest-0 ... linking ... <interactive>: /home/heatsink/.cabal/lib/WeakTest-0/ghc-6.10.3/HSWeakTest-0.o: unknown symbol `weak_test'

I encountered this problem while trying to build a package that contains C++ code. Because GCC generates weak symbols when compiling C++, libraries that contain C++ code will not work in GHCi. (Granted, this is not the only problem with mixing C++ and Haskell.)