Hamlet quasiquoter (Haskell): Allowed syntax in if-elseif-else expressions

Question

The Hamlet quasiquoter allows if-then-else syntax like

  [hamlet|
   $if 3 < 4
     Three is less than 4.
   $elseif "a" == "b"
     'a' equals 'b'.
   $else
     Nothing worked out.
  |]

However, I find that if I replace the test "a" == "b" with 'a' == 'b' I get an error complaining that 'a' is an illegal variable name.

So what syntax is and is not allowed in an $if or $elseif test? I have not been able to find this documented anywhere.