implicit parameters

There's apparently something very basic that I don't understand about implicit parameters. In the following program, compilation fails if either of the type declarations is omitted. What is "implicit" about implicit parameters, then? {-# OPTIONS -fglasgow-exts #-} main = print f1 f1 = let ?f = \ x -> x ++ x in f2 f2 :: (?f :: String -> String) => String f2 = f3 f3 :: (?f :: String -> String) => String f3 = ?f "x"