Sorry, left out an important verb, *hide*:

You can the prelude and thus the Ord class and make your own < and >

You can *hide* the prelude and thus the Ord class and make your own < and > Jared. -- http://www.updike.org/~jared/ reverse ")-:"

Am Mittwoch, 26. Juli 2006 16:20 schrieb Dmitri O.Kondratiev:

On 7/25/06, Jared Updike wrote:

...

...

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

The Haskell98 way to give the type is greaterMyOrd :: (MyOrd a, MyOrd 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.

As Janis Voigt mentioned, this sort of constraint needs extensions because it's not Haskell98

-- 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?

hugs infers MyOrd> :t greaterMyOrd greaterMyOrd :: (MyOrd a, MyOrd b) => (b,a) -> (b,a) -> Bool as Janis pointed out, ghc doesn't do complete context reduction, I don't know why either. It also happens for standard classes, e.g. Prelude> let equ xs@(_:_) ys = xs == ys; equ [] ys = null ys Prelude> :t equ equ :: (Eq [a]) => [a] -> [a] -> Bool

-- 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>

Janis already answered that

-- I am lost. Please enlight me, what am I doing wrong trying to create my own class, its instance and then using it. Thanks !

another problem is that you did not specify any instances of MyOrd, so up to now there aren't any: *MyOrd> greaterMyOrd (True, 'b') (False, 'c') <interactive>:1:0: No instances for (MyOrd Bool, MyOrd Char) arising from use of `greaterMyOrd' at <interactive>:1:0-11 Probable fix: add an instance declaration for (MyOrd Bool, MyOrd Char) In the definition of `it': it = greaterMyOrd (True, 'b') (False, 'c') note that here ghci doesn't ask for an instance MyOrd (Bool, Char), but for separate instances MyOrd Bool and MyOrd Char and even if you provide an instance MyOrd Integer (or Int, ...), you'll still need to give expression type signatures, because defaulting does only work with standard classes (cf. the Haskell-Report, sect. 4.3.4) *MyOrd> 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>:1:16 Probable fix: add a type signature that fixes these type variable(s) *MyOrd> greaterMyOrd (2 :: Integer,3 :: Integer) (1,2) True Cheers, Daniel -- "In My Egotistical Opinion, most people's C programs should be indented six feet downward and covered with dirt." -- Blair P. Houghton

Bulat, Dmitri is not necessarily "mixing [...] class and its (default) instance". Bulat, one could suspect a reasonable use of default class methods: http://www.haskell.org/onlinereport/decls.html#overloading Indeed, Dmitri's declarations make sense as default class methods. ... except for Jared's observation: the attempt to rely on Ord. If %< in place of < was used on the RHSs, then this would really look like default methods. Ralf PS: We don't want to send people using overlapping instances more often than necessary :-)

-----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe- bounces@haskell.org] On Behalf Of Bulat Ziganshin Sent: Tuesday, July 25, 2006 9:46 AM To: Dmitri O.Kondratiev Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Newbie Q: Deriving MyOrd from Eq problem

Hello Dmitri,

Tuesday, July 25, 2006, 8:15:41 PM, you wrote:

...

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)

you are mixing definition of class and its (default) instance. try the following:

class Eq a => MyOrd a where (%<=), (%>), (%>=) :: a -> a -> Bool

instance Ord a => MyOrd a where x %<= y = (x < y || x == y) x %> y = y < x x %>= y = (y < x || x == y)

although i don't think it is what you want. actually Haskell don't have a good way to define default instance for some subclass, although there are some proposals how this can be addressed. you should either define exactly one instance for all "Ord"ed types or duplicate this trivial definition in every instance. in GHC you also has an option to use "overlapping" instances but this can work only if all your other "instance" definitions don't use deriving from typeclasses. i.e. the following is prohibited:

class Eq a => MyOrd a where ... instance Ord a => MyOrd a where ... instance SomeClass a => MyOrd a where ...

and even if you will be accurate, i'm not sure that "overlapping" will work properly

ps: you can ask me in Russian via private mail

-- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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