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tcfail072.hs:23:13: error:
• Could not deduce (Ord p0) arising from a use of ‘g’
from the context: (Ord p, Ord q)
bound by the type signature for:
g :: forall p q. (Ord p, Ord q) => AB p q -> Bool
at tcfail072.hs:22:1-38
The type variable ‘p0’ is ambiguous
These potential instances exist:
instance Ord Ordering -- Defined in ‘GHC.Classes’
instance Ord Integer -- Defined in ‘GHC.Num.Integer’
instance Ord () -- Defined in ‘GHC.Classes’
...plus N others
...plus N instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the expression: g A
In an equation for ‘g’: g (B _ _) = g A
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