Am Samstag 02 Mai 2009 20:00:54 schrieb Rahul Kapoor:
OK, I think what you're saying is to work with (random) integers and use fromEnum and toEnum to get corresponding DayOfWeek. But I get this when I try to use toEnum:
*Main> toEnum 2
ghci does not know what type of enum you want to create from the number 2. Try: toEnum 2 :: DayOfWeek
That said, I would expect "toEnum 2" to give an error like: 'Ambiguous type variable `a'....'. So I am not sure why your error message says: '** Exception: Prelude.Enum.().toEnum: bad argument'
Defaulting. If ghci doesn't know which type (let's call it a) is demanded, it looks at the constraints (here Enum a) and tries to choose a type from its default list that satisfies the constraints. AFAIK, the default list is (), Integer, Double. () satisfies the constraint, so ghci chooses that. fromEnum () is 0, so 0 is the only acceptable argument for toEnum :: Int -> (). It's a two-edged sword, if ghci didn't default, it would have to issue a *lot* of ambiguous type messages and you would have to give a type signature to most expressions you type at the prompt, with the defaulting, you get pretty unintuitive error messages when it does the wrong thing.