This library provides a pure Haskell complex-to-complex Fast Fourier Transform implementation for arbitrary length input vectors, using a mixed-radix decimation-in-time algorithm with specialised straight-line code for a range of base transform sizes, Rader's algorithm for prime length base transforms, and an empirical optimisation scheme to select a good problem decomposition. This package is probably primarily of pedagogical interest (FFTW is about five times faster for most input sizes). There is a long series of blog articles describing the development of the package, indexed at http://www.skybluetrades.net/haskell-fft-index.html.