I have the following problem with typeclasses : I have created a type class in a manner similar to the following module Dummy where import Char class (Show t) => Dummy t where kite :: t -> t instance (Dummy t) => Dummy [t] where kite = reverse instance Dummy String where kite = map toUpper And use it thus : import Dummy main :: IO () main = do print.kite $ "Rest in peace" If I compile without any option, I get as expected an error because of the type synonym instance. If I remove the instance for String, I also get as expected an error because Char is not an instance of Dummy. But if I add (as suggested in the first error message) the GHC's option -XTypeSynonymInstances, I get an error because of overlapping instances, even if Char is still not an instance of Dummy and I don't see how String could be eligible for the (Dummy t) => Dummy [t] instance... Thanks in advance for your help, ARJANEN Loic.
You can add OverlappingInstances to allow for both [a] and [Char]
intstances be used. It will choose the least general instance for
you.
On Wed, Jul 13, 2011 at 8:56 AM, ARJANEN Loïc Jean David
I have the following problem with typeclasses : I have created a type class in a manner similar to the following
module Dummy where import Char class (Show t) => Dummy t where kite :: t -> t instance (Dummy t) => Dummy [t] where kite = reverse instance Dummy String where kite = map toUpper
And use it thus :
import Dummy main :: IO () main = do print.kite $ "Rest in peace"
If I compile without any option, I get as expected an error because of the type synonym instance. If I remove the instance for String, I also get as expected an error because Char is not an instance of Dummy. But if I add (as suggested in the first error message) the GHC's option -XTypeSynonymInstances, I get an error because of overlapping instances, even if Char is still not an instance of Dummy and I don't see how String could be eligible for the (Dummy t) => Dummy [t] instance...
Thanks in advance for your help, ARJANEN Loic.
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On Wed, Jul 13, 2011 at 7:56 AM, ARJANEN Loïc Jean David
If I compile without any option, I get as expected an error because of the type synonym instance. If I remove the instance for String, I also get as expected an error because Char is not an instance of Dummy. But if I add (as suggested in the first error message) the GHC's option -XTypeSynonymInstances, I get an error because of overlapping instances, even if Char is still not an instance of Dummy and I don't see how String could be eligible for the (Dummy t) => Dummy [t] instance...
This is one of the trickier bits about Haskell typeclasses. The instance:
instance (Dummy t) => Dummy [t]
can be read in English as: "The type [t] is an instance of Dummy. Also, using this instance is a compilation failure if t is not an instance of Dummy." More succinctly, the information to the left of the (=>) is not used at all in instance resolution. It does, however, bring the constraint into scope in the body of the instance declaration, so it isn't useless. Antoine
Thanks in advance for your help, ARJANEN Loic.
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participants (3)
-
Antoine Latter -
ARJANEN Loïc Jean David -
David McBride