I'm not a Haskell user at all, and I cannot test it, but I would say you just have to provide the path of the socket file in this string.
Instead of:
connectPort = PortNumber 6379
you would have:
connectPort = UnixSocket "/tmp/redis.sock"
Of course, /tmp/redis.sock should be declared in the server-side Redis configuration file using the following parameters:
# Specify the path for the unix socket that will be used to listen for
# incoming connections. There is no default, so Redis will not listen
# on a unix socket when not specified.
#
unixsocket /tmp/redis.sock
unixsocketperm 755
Please note by default, unix domain socket parameters are commented out.