[Haskell-beginners] Calculating SHA1 hashes?

I'm trying to figure out the best way to calculate SHA1 hashes for files using Haskell. There are several libraries I tried from Hackage, but they all seem to end up computing the hash far too slowly vs. OpenSSL. Some of the libraries seem to be FFI bindings to OpenSSL, but I haven't figured out how to efficiently feed these. (For instance, hopenssl consumes a [Word8], but I haven't figured out how to lazily produce that from a file; using "unpack" on a ByteString seems to end up much too slow.) I also tried mimicking nano-md5 by writing a "nano-sha1" that was largely a "replace the name of the digest function" hack job, but I couldn't manage to get that working. Is there a particular accepted way to generate hashes that I'm missing?