[icey]

Gauche Scheme has a bcrypt implementation, but I don't know what the compatibility story is between mzscheme and Gauche. I think they're both R5RS compliant, so it should work.

I see that newer versions of Arc run on Racket, but I have no idea if that's what HN is using or not.

I haven't seen a scheme powered PBKDF2 implementation so I'd guess that's out.

The only other expensive KDF I can think of is scrypt, but I would be pretty surprised if that's got a scheme implementation.

Of course, I guess pg could have decided to call out to the OS to run any of those functions too.