Please, what is wrong here?
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module T where
class S a where s :: a -> a
class S a => OS a where o :: a -> a
class S a => R a where r :: a -> a
data F a = F a a
instance R a => S (F a) where s x = x
instance (R a,OS a) => OS (F a) where o x = x
instance (R a,OS a,Num a) => S (F a) where s _ = error "" --
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ghc -c -fglasgow-exts -optC-fallow-overlapping-instances T.hs
reports
T.hs:10:
Could not deduce `Num a'
(arising from an instance declaration at T.hs:10)
from the context (R a, OS a)
Probable cause: missing `Num a' in instance declaration context
When checking the superclasses of an instance declaration
This is ghc-3.02-i386-unknown-bin... taken from
dcs.gla.ac.uk..../glasgow/3.02
The idea was that the second S (F a) is defined more specifically.
OS (F a) uses the instance of S (F a). It has to choose the one
which is more specific for the given data - ?
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Sergey Mechveliani
[EMAIL PROTECTED]
