Readme for OpenCLWrappers-0.0.0.1
You must link explicitly to opencl library, name of which varies.
eg.
ghc -lamdocl64 --make opencl.hs
ghc -lOpenCL --make opencl.hs
Some examples can be found in http://users.tkk.fi/~jkarlson/codes/OpenCLWrappers/examples