[Haskell-beginners] Fwd: Type constraints question

18 Dec 2009

      Apologies for the spam, I had some difficulty getting my email address set
up right on the list and wanted to make sure this finally got through (or
perhaps there are like 5 copies on the list now... sorry.)

thanks

---------- Forwarded message ----------

Ok, now I will try asking the right question (previous try wasn't actually
an example of the problem I'm having)

Here's the code:

data Test = Test

data Test2 = Test2

class MyClass a where
    getChild :: MyClass b => a -> b

instance MyClass Test where
    getChild a = Test2

instance MyClass Test2 where
    getChild a = Test

On HUGS I get:

Error occurred
ERROR line 9 - Inferred type is not general enough
*** Expression    : getChild
*** Expected type : (MyClass Test, MyClass a) => Test -> a
*** Inferred type : (MyClass Test, MyClass Test2) => Test -> Test2

So, what's the problem with always returning a particular instance of
MyClass? I just want to constrain it to be an instance of MyClass, nothing
more.

thanks,

David

2009/12/18 David Gordon 
...
Hi Folks,
Total newbie here. I don't know if I am having a syntactic problem or a
conceptual problem.
...
This code:
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
results in:
[1 of 1] Compiling Main             ( test.hs, interpreted )
test.hs:10:15:
    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'
test.hs:13:15:
    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... ;)
many thanks,
David