because all types are really (or <x> null).
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Parse it as lisp.
It's haskell-type-system-speak for "If you declare something to be a String, it really means String or null."
The Haskell type for that would be "Maybe String". When you get a Maybe value, the compiler gives an error if you don't deal with the possibility that the value might be Nothing.
I don't think he did... took me a couple of passes, but it parsed in the end.