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<interactive>:1:1: error:
• Ambiguous type variable ‘a0’ arising from a use of ‘foo’
prevents the constraint ‘(Num a0)’ from being solved.
Probable fix: use a type annotation to specify what ‘a0’ should be.
These potential instances exist:
instance Num Integer -- Defined in ‘GHC.Num’
instance Num Double -- Defined in ‘GHC.Float’
instance Num Float -- Defined in ‘GHC.Float’
...plus two others
...plus 8 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the expression: foo
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