There are a couple of known bugs in Hugs which aren't mentioned on the bugs page, or in the documentation section on the differences between Hugs98 and Haskell 98. 1. Hugs doesn't support Rational constants: Prelude> 1.0e27 :: Rational 999999988484154753734934528 % 1 2. Hugs's implementation of the monomorphism restriction differs from the spec (discussed a few months back). Also, the documentation still claims that some libraries don't exist (Directory, Time, CPUTime), and mentions some other libraries which aren't part of Haskell 98 (Bit, Nat, Signed). Cheers, Simon