[Haskell-beginners] Type constraints question
david.robert.gordon at googlemail.com
Fri Dec 18 05:54:03 EST 2009
Total newbie here. I don't know if I am having a syntactic problem or a
data Test = Test
data Test2 = Test2
class MyClass a where
getChild :: (MyClass b) => a -> b
instance MyClass Test where
getChild = Test2
instance MyClass Test2 where
getChild = Test
[1 of 1] Compiling Main ( test.hs, interpreted )
Couldn't match expected type `Test -> b'
against inferred type `Test2'
In the expression: Test2
In the definition of `getChild': getChild = Test2
In the instance declaration for `MyClass Test'
Couldn't match expected type `Test2 -> b'
against inferred type `Test'
In the expression: Test
In the definition of `getChild': getChild = Test
In the instance declaration for `MyClass Test2'
Failed, modules loaded: none.
Is this a reasonable thing to try and do in Haskell? If not, I have a lot
more questions... ;)
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