for example: I will ask the glorious compiler, and it says: *Main> :t calcFFT calcFFT :: (Math.FFT.Base.FFTWReal r) => V.Vector (Complex r) -> V.Vector (Complex r) I then put the signature in my code, reload it, and: Not in scope: type constructor or class `Math.FFT.Base.FFTWReal' It seems very strange to me that the fully qualified module name doesn't work, nor does any combination thereof, e.g. FFT.FFTWReal, FFT.Base.FFTWReal, etc... I'm importing FFT as: import qualified Math.FFT as FFT I'm not sure if that causes the problem. Generally speaking this happens to me quite a lot, and I've never tried to understand what's going on, because the signature is not required. I've decided to try now :-) Obviously I'll need some help. Thanks, Brian