On Sun, 6 Sep 2020, Ignat Insarov wrote:
I have a problem trying to do something very simple.
Consider this conversation with GHC:
λ data Y α = Y {y ∷ α} λ defaultY = Y {y = mempty} λ :type defaultY defaultY :: Monoid α => Y α λ :type defaultY {y = "c"} ∷ Y String
<interactive>:1:1: error: • Ambiguous type variable ‘α0’ arising from a use of ‘defaultY’ prevents the constraint ‘(Monoid α0)’ from being solved. Probable fix: use a type annotation to specify what ‘α0’ should be. These potential instances exist: instance Monoid a => Monoid (IO a) -- Defined in ‘GHC.Base’ instance Monoid Ordering -- Defined in ‘GHC.Base’ instance Semigroup a => Monoid (Maybe a) -- Defined in ‘GHC.Base’ ...plus 7 others (use -fprint-potential-instances to see them all) • In the expression: defaultY In the expression: defaultY {y = "c"} :: Y String
Why does this not work?
I think it should work: :type (defaultY::Y [()]) {y = "c"} The problem is, that GHCi cannot infer the type of defaultY, because the update changes the type. By adding the type annotation Y String at the end, you only tell GHCi what it already knows, namely, that the result of the update has type Y String. But there is no way to infer what the type of 'mempty' must have been before the update.