Haskell-Conceal also works a lot better than unicode-haskell: it doesn't erroneously turn "\n" into "λn", and it replaces :: with a single-glyph equivalent so that signature and definition line up nicely.

Having said that, I still prefer <- and -> to ← and →, at least on screen: those Unicode arrowheads are too small on all the fixed-width fonts I've ever seen.