On 7/25/06, Jared Updike
I am trying to derive MyOrd class from Eq (Prelude):
class Eq a => MyOrd a where (%<=), (%>), (%>=) :: a -> a -> Bool x %<= y = (x < y || x == y) x %> y = y < x x %>= y = (y < x || x == y)
Q: What's wrong? Why 'Ord' gets into play here?
You are using < which is a function on types that instance the class Ord, so the compiler is telling you to add (Ord a) to the same place you have (Eq a) or don't use < or > or any function in the class Ord. You can the prelude and thus the Ord class and make your own < and > functions but you can't make them refer to the "real" < and > functions without Ord because that is where they live.
Jared. -- http://www.updike.org/~jared/ reverse ")-:"
-- Ok, then I can derive MyOrd class directly from Ord: class Ord a => MyOrd a where (%<), (%<=), (%>), (%>=) :: a -> a -> Bool x %< y = x < y x %<= y = (x < y || x == y) x %> y = y < x x %>= y = (y < x || x == y) instance (MyOrd a, MyOrd b) => MyOrd (a,b) where (x1, y1) %< (x2, y2) = (x1 %< x2) && (y1 %< y2) (x1, y1) %> (x2, y2) = (x1 %> x2) && (y1 %> y2) (x1, y1) %<= (x2, y2) = (x1 %<= x2) && (y1 %<= y2) (x1, y1) %>= (x2, y2) = (x1 %>= x2) && (y1 %>= y2) greaterMyOrd :: (MyOrd (a, b)) => (a, b) -> (a, b) -> Bool greaterMyOrd (x,y) (z,t) = (x,y) %> (z,t) -- This should work, right? Yet I get this error message: ClassTest.hs:39:0: Non-type variables in constraint: MyOrd (a, b) (Use -fglasgow-exts to permit this) In the type signature: greaterMyOrd :: (MyOrd (a, b)) => (a, b) -> (a, b) -> Bool Failed, modules loaded: none. -- Notwithstanding :) when i comment out declaration: -- greaterMyOrd :: (MyOrd (a, b)) => (a, b) -> (a, b) -> Bool -- program gets compiled and checking type of 'greaterMyOrd' gives: *ClassTest> :t greaterMyOrd greaterMyOrd :: (MyOrd (a, b)) => (a, b) -> (a, b) -> Bool -- which is the same as I tried to declare in the program source. What is hapenning here? -- Now, when trying to use function 'greaterMyOrd' I get: *ClassTest> greaterMyOrd (2, 3) (1, 2) <interactive>:1:0: Ambiguous type variable `a' in the constraints: `MyOrd a' arising from use of `greaterMyOrd' at <interactive>:1:0-11 `Num a' arising from the literal `2' at <interactive>:1:14 Probable fix: add a type signature that fixes these type variable(s) <interactive>:1:0: Ambiguous type variable `b' in the constraints: `MyOrd b' arising from use of `greaterMyOrd' at <interactive>:1:0-11 `Num b' arising from the literal `3' at <interactive>:1:17 Probable fix: add a type signature that fixes these type variable(s) -- Then I try to declare argument types explicitely and get this: *ClassTest> greaterMyOrd (2::MyOrd, 3::MyOrd) (1::MyOrd, 2::MyOrd) <interactive>:1:17: Class `MyOrd' used as a type In an expression type signature: MyOrd In the expression: 2 :: MyOrd In the first argument of `greaterMyOrd', namely `(2 :: MyOrd, 3 :: MyOrd)' *ClassTest> -- I am lost. Please enlight me, what am I doing wrong trying to create my own class, its instance and then using it. Thanks ! -- Dmitri O Kondratiev, dokondr@gmail.com http://www.geocities.com/dkondr/