Re: [Haskell-cafe] Hspec + QuickCheck passing type

28 Mar 2016

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Martijn Rijkeboer  writes:
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Unfortunately this doesn't work, since QuickCheck doesn't know which
Arbitrary instance to use. How can I pass the type, e.g. x::Type1, to this
function so QuickCheck knows what Arbitrary instance to use?
For example:

    {-# LANGUAGE ScopedTypeVariables #-}

    import Test.QuickCheck
    import Test.Hspec
    import Data.Serialize
    import Data.Proxy

    typeTest :: forall a. (Show a, Eq a, Arbitrary a, Serialize a)
             => String -> Proxy a -> SpecWith ()
    typeTest name _ =
      context name $
        it "decode inverses encode" $ property $
          \(x :: a) -> (decode . encode) x == Right x

    main :: IO ()
    main = hspec $ typeTest "foo" (Proxy :: Proxy Int)

The use of Proxy conveys the needed type information, as Will had also
suggested.

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