[Haskell-cafe] Cannot update a field in a record with a polymorphic type.

Ignat Insarov kindaro at gmail.com
Sun Sep 6 09:47:58 UTC 2020


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?

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