Neil Mitchell wrote:
Hi
isBool x = isLT x || isGT x isNum x = not $ isBool x
isLT and isGT can be derived automatically using derve [1], with the Is class (or DrIFT if you want).
You can also get a long way with GHC's built in derivations for Eq, Enum and Show. If an Enum instance is possible then you can write something like isBool = (`elem` [LT .. GT]) isNum = (`elem` [Minus .. Mul]) If enum is not possible, because some of the constructors are not nullary, then there is the following cute/ugly hack if you instead derive Show: constrName = takeWhile (/=' ') . show and you can say isBool x = constrName x `elem` ["LT","GT"] isNum x = constrName x `elem` ["Minus","Plus","Mul"] What this leads us towards is that it might be rather nice (perhaps even nice enough to build into a compiler) to be able to derive, for each type with multiple constructors, a type 'which is the enumeration of the constructors'. I.e. a type with the same constructors (up to some namespace fix like prepending with C) but all nullary: data Atom = Null | MyInt Int | MyString String derives... data AtomCons = CNull | CMyInt | CMyString deriving (Ord,Eq,Enum,Show) and a function constr :: Atom -> AtomCons Jules